feat(coreutils-port): add module-vendor mode with manifest and drift CI

[chaliy]

Summary

Adds module-vendor mode to bashkit-coreutils-port — a port-module
subcommand that vendors platform-clean uucore modules verbatim into
crates/bashkit/src/builtins/generated/<out>/, gated by a
vendored.toml manifest declaring per-import substitution policy.
Args mode is unchanged. Closes #1533. The first consumer (uucore::format
for the printf migration) lands separately as #1534.

What

• port-module <UUTILS_DIR> <MODULE> [<REV>] subcommand. Existing
  positional CLI (args mode) is preserved unchanged.
• Source split: main.rs (dispatcher) → args.rs, module.rs,
  manifest.rs. Args logic relocates verbatim.
• Policy enforcement: walks every top-level use, flattens nested
  groups. Hard errors on Fluent / uucore::translate / uucore::i18n.
  Every other uucore-internal use must match a manifest substitution
  (unmatched aborts the port — silent emission of broken imports is
  rejected).
• Substitution actions: error is fully implemented; inline /
  replace_with are accepted in the schema but error at runtime — they
  need the future syn-based import rewriter, which the first real
  consumer (Vendor uucore::format and migrate printf.rs to use it #1534) will drive.
• vendored.toml lives next to the tool (the tool owns the manifest;
  the drift workflow only reads it). Initial manifest is empty.
• coreutils-args-drift.yml extends to iterate the manifest and
  re-port every vendored module against uutils HEAD; args drift and
  module drift now land in a single auto-PR.
• Spec updates: specs/coreutils-args-port.md gains a Module Mode
  section. specs/maintenance.md broadens the drift checklist.
  AGENTS.md spec-table description updated.

Why

Verification on a probe branch ruled out depending on uucore at
runtime: +98 s cold build, declared-but-unused 9 KB binary growth,
and a hard wasm32 break (uucore → rustix → errno). Vendoring
selected modules at port-time using the same machinery (and drift CI)
that args mode uses today gives reviewable, grep-able output and
predictable build times — at the cost of needing to re-run the recipe
on every uutils bump (the drift workflow handles that).

Tests

13 unit tests in bashkit-coreutils-port covering:

• Happy path (external imports pass through verbatim with banner)
• Fluent / uucore::translate / uucore::i18n hard errors
• Unresolved uucore-internal imports
• error action policy rejection
• replace_with / inline rewriter-not-implemented errors
• Module-not-in-manifest
• Recursive directory walking
• Use-tree group flattening (use uucore::{a, b})
• Manifest schema parsing + unknown-action rejection

Test plan

• cargo test -p bashkit-coreutils-port — 13/13 green
• cargo fmt --check clean
• cargo clippy --all-targets -- -D warnings clean across workspace
• CLI smoke: port-module usage prints, missing module surfaces
  manifest path
• YAML lint of updated drift workflow
• CI green on this PR

Closes #1533

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