fix(cli): version-banner placeholders + doctor duplication check + install docs

[HenryLach]

Three related changes packaged together because they all spring from the same operator-experience surface: Pi 0.75.0 (2026-05-17) moved user-scoped pi extension installs from npm's global root to a private dir at ~/.pi/agent/npm/node_modules/, and the change wasn't reflected in Taskplane's docs or its diagnostics.

1. taskplane --version no longer shows vundefined, initialized unknown

The previous cmdVersion() unconditionally interpolated info.version and info.installedAt from .pi/taskplane.json even when the marker file didn't have those fields, producing:

Config:   .pi/taskplane.json (vundefined, initialized unknown)

This was reproducible in any repo where .pi/taskplane.json exists for some other purpose (e.g. taskplane's own source repo where the file carries only migration history rather than init metadata). The fix defensively renders only the fields that are actually present and falls through to (metadata only) when neither field is set.

2. taskplane doctor now detects two-copy duplication

New helper detectDuplicateTaskplaneInstall() identifies the state where users have both the Pi-private install (refreshed by pi update) and a system npm-global install (refreshed by npm install -g taskplane). When the two differ in version, the system-wide CLI silently drifts behind while pi update reports success — which is the failure mode this PR was born from.

Sample doctor output in the duplication state:

  ⚠️  taskplane is installed in TWO locations with different versions:
       Pi-private: v0.30.2  (C:\Users\HenryLach\.pi\agent\npm\node_modules\taskplane)
       npm-global: v0.30.1  (C:\Users\HenryLach\AppData\Local\mise\installs\node\24.15.0\node_modules\taskplane)
       → `pi update` only refreshes the Pi-private copy.
       → Recommended fix: drop the npm-global copy and put Pi's bin dir on PATH:
           npm uninstall -g taskplane
           export PATH="$HOME/.pi/agent/npm/node_modules/.bin:$PATH"  # add to ~/.bashrc / ~/.zshrc

Same-version duplications are silently tolerated (not yet problematic; will drift later but isn't a problem in the present tense). Best-effort: silently returns null if npm root -g is unreachable.

3. README.md + docs/tutorials/install.md updates

• README installation section — documents the PATH requirement for Pi 0.75.0+ (bash/zsh + PowerShell snippets) so taskplane <cmd> works on the shell after pi install npm:taskplane. Adds the duplication caveat for users who previously ran npm install -g taskplane under older docs.
• Install tutorial — reframes Option A to explicitly steer users away from npm install -g taskplane as the primary install command, citing the Pi 0.75.0 install-location change. Expands the 'taskplane not on PATH' section into three labeled options (npx, direct bin invocation, PATH addition) with platform-specific snippets.

Validation

Gate	Result
npm run typecheck	✅ pass
npm run lint	✅ 286 warnings / 671 infos — identical to main
npm run format:check	✅ pass (1 biome auto-format applied to bin/taskplane.mjs)
Full test suite	✅ 3,714 / 3,715 pass, 1 skipped — zero new failures
taskplane --version	✅ shows (metadata only) instead of (vundefined, initialized unknown)
taskplane doctor	✅ all checks pass; duplication check correctly silent in this repo (only Pi-private exists post-cleanup)

Note on release

This is a doctor + docs change; the CLI behavior changes are display-only and have no impact on /orch or /task execution. No release needed unless paired with other fixes. Will sit in [Unreleased] on the CHANGELOG and ship with the next batch.