chore: strip instruction surfaces for the unhobble Fable 5.1 baseline (#4090)

Phase 2 (bare) of /claude-config:unhobble, experiment
claude-code-plugins-fable-5-1-20260908-15744de6. Strips five rules and the
AGENTS.md "Validate a change" section, trims the skill-bodies rule, re-renders
the rules index, and adds the durable experiment record under
.claude/unhobble/<experiment-id>/. The observe window runs on main from this
merge; the readd phase restores the register hold and ledger-defended surfaces
from c41c642.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nc42dgu8oeNnjj1xwo9sNo

Author: kyle-sexton

.claude/rules/catalog-taxonomy.md

@@ -1,46 +1,18 @@
 ---
-description: "Skill and agent bodies state the current rule and its reason, never the incident, PR, or model that motivated it, and name their successor in a `## Next` section; read before editing any skill body"
+description: "Skill and agent bodies carry a four-part verification record for any volatile specific they restate, and name their successor in a `## Next` section; read before editing any skill body"
 paths:
   - "plugins/*/skills/**"
   - "plugins/*/agents/**"
 ---
 
 # Skill bodies state current rules
 
-A skill body is read by the model on every invocation. Its authority is the behavior it
-prescribes, not the incident that motivated it, so the body carries the rule and the reason and
-nothing about how the rule got there. This follows the bundled `/claude-api prompt-audit` guide
-(Group 2, "Brittle skill files").
-
-Keep out of a skill or agent body:
-
-- Issue and pull-request numbers from this repository, incident IDs, and past-tense narration of
-  why a rule exists ("this was added after ...", "an earlier version claimed ...").
-- Pinned model names in behavioral guidance. A rule that only holds on one model belongs in the
-  `playbooks` model-adaptation chapters, not in a skill body.
-- Date-conditional guidance ("before 2026-08 ...", "until version X ships ...").
-- Hardcoded paths, flags, and version numbers stated as bare fact with no verification.
-
-Keep in the body:
-
-- The rule, stated in the present tense, with the reason beside it.
-- A pointer to an external upstream source (an official doc page, an upstream issue) when the rule
-  restates a volatile specific it cannot defer to at read time, recorded as the four-part
-  verification record the
-  [upstream-drift convention](../../docs/conventions/upstream-drift/README.md) defines: claim,
-  basis, as-of date, recheck trigger. A dated verification with a trigger is the correct form; an
-  undated claim is the defect.
-
-History belongs in the plugin's `CHANGELOG.md`, the commit message, and `docs/adr/`. A reader who
-needs the archaeology finds it there; the model reading the skill does not need it to act.
-
-The platform skill-authoring guidance recommends the opposite shape for superseded guidance: an
-in-body "Old patterns" section inside a collapsed `<details>` block
-([content guidelines](https://platform.claude.com/docs/en/agents-and-tools/agent-skills/best-practices#content-guidelines),
-verified 2026-09-10). This repository deviates on purpose: every body line is a recurring token
-cost on invocation, so history routes to the CHANGELOG, the commit, and `docs/adr/`, and a volatile
-specific carries the four-part record above instead of a legacy stanza. Recheck this paragraph when
-that page drops or changes the "Old patterns" recommendation.
+A pointer to an external upstream source (an official doc page, an upstream issue) is required
+when a skill or agent body restates a volatile specific it cannot defer to at read time, recorded
+as the four-part verification record the
+[upstream-drift convention](../../docs/conventions/upstream-drift/README.md) defines: claim,
+basis, as-of date, recheck trigger. A dated verification with a trigger is the correct form; an
+undated claim is the defect.
 
 ## Successor sections
 

.claude/rules/hook-budget.md

@@ -0,0 +1,38 @@
+# unhobble experiment: Fable 5.1 bare baseline
+
+Durable mirror for one run of `/claude-config:unhobble` (experiment id
+`claude-code-plugins-fable-5-1-20260908-15744de6`). The snapshot ran in an ephemeral cloud
+container; that container goes away when the session that opened pull request #4090 ends, and
+the plugin-data copy of the manifest goes with it, so from then on this directory is the
+canonical experiment record, not a copy. It lives under `.claude/` rather than the topic-docs
+contract slice because the contract-slice gate red-lines any pull request that carries a slice,
+and this record has to outlive the whole observe window.
+
+| File | Role |
+|---|---|
+| `stumbles.md` | The observation ledger. Append a row whenever the bare model stumbles, or does something better. |
+| `manifest-mirror.json` | The Phase 1 manifest with absolute paths tokenized: every surface, its class, what was stripped, how to restore it, and the four resolved decisions. |
+| `evidence/research-D*.md` | Source-backed research memos behind decisions D1 to D3 (official docs fetched 2026-09-11). |
+| `evidence/decision-*.md` | Two independent decision agents' verdicts, one on Fable 5.1 and one on Opus 5, which agreed on all four decisions. |
+
+Observe phase: on 2026-09-12 the operator decided to merge #4090 so the window runs on main;
+from that merge on, the bare state is the state of main (the manifest records this under
+`checkout.merged_to_main`). Work normally on real tasks from fresh sessions on main and append a
+row here whenever the model stumbles or does something better without an instruction.
+
+Restoring the canonical state before readd: the skill compares `checkout.worktree_path` with the
+resolved absolute checkout before every phase command and aborts on a mismatch, so the mirror
+cannot be copied back verbatim. Copy `manifest-mirror.json` to
+`${CLAUDE_PLUGIN_DATA}/unhobble/<experiment-id>/manifest.json` with the `<worktree>` token
+replaced by the output of `git rev-parse --show-toplevel` and the `<CLAUDE_PLUGIN_DATA>` token
+replaced by the resolved plugin data directory, copy `stumbles.md` alongside it, and create an
+empty `backups/` directory next to them. Keep the tokenized copy here unchanged.
+
+Readd phase: start a fresh branch off main, restore `.claude/rules/pr-body-contract.md` from
+`git show c41c6422:.claude/rules/pr-body-contract.md` regardless of the ledger (register hold,
+contested external-publication), restore only what the ledger defends with repeated same-cause
+rows, re-render the AGENTS.md rules index with
+`plugins/instruction-placement/scripts/render-index.sh write --file AGENTS.md`, set
+`"phase": "closed"` in the manifest, and close the experiment. Every stripped surface is
+recoverable from commit `c41c6422` (the pre-strip base); the manifest names each one. Whether this
+directory is kept as the experiment's record or removed at close is the readd phase's call.