Implement per-pack command populator step in installer engine

[virtualian]

Context

Implementation of the recommendation from issue #146's investigation. See:

• Report: reports/146-pack-command-staging-investigation.md (on main)
• Investigation PR: Add issue #146 investigation report — pack command staging gap #164 (merged)

Today engine/command-migration.ts symlinks ~/.claude/commands/<name>.md → ~/.pai/commands/<name>.md but nothing populates ~/.pai/commands/ from the pack source. Result: pack-shipped commands (e.g. cs.md from ContextSearch) never reach the runtime.

Recommended approach

Per-pack populator that walks Packs/*/src/commands/<name>.md on every install, with per-file copy-if-not-exists semantics (preserves user edits; lets new pack commands land on existing installs).

Acceptance

• New file at Releases/v4.0.3+/.claude/PAI-Install/engine/pack-source-staging.ts (or extension of an existing module in that same directory) walks Packs/*/src/commands/<name>.md, copies each into ~/.pai/commands/<name>.md only if dest does not exist
• Releases/v4.0.3+/.claude/PAI-Install/engine/actions.ts:runRepository invokes the new step BEFORE migratePerPackCommands
• Soft-fail per file (matches existing migrator pattern)
• Idempotent: second run on an unchanged install does no writes
• Test fixture covers: fresh install (populator creates files); upgrade with user-edits (populator skips); upgrade shipping new command (populator stages new file)

Out of scope

• Skill-side analogue (Resync Research and Learning pack sources into ~/.pai/skills/ runtime #145)
• Generalised pack-source-walker (decide after the commands case is in hand)
• If v4.0.4+ is cut before implementation: rebase onto whichever Releases path is current

Refs

Closes #146 when merged. Refs #145 (skill-side analogue), #144 (two-root restructure dependency), PR #164 (investigation).