[Plugins] Installer, lock file, and rollback (path + git sources) #1070

Part of #1068.

Scope

Implement the install pipeline for path and git sources, generate a plugin.lock.json, and provide full rollback via uninstall.

Requirements

• squad plugin install <source> [--ref <ref>] supports path and git sources.
• Install request fields (source, ref, optional name pin, optional permission acks) are persisted under .squad/plugins/<name>/requested.json (or equivalent).
• Generate plugin.lock.json containing:
  • Resolved immutable ref + content digest.
  • File inventory with per-file hashes.
  • Effective permissions (derived from declared components).
  • Install timestamp + installer version + source provenance.
• Atomic install: either the full install succeeds or no files are left behind.
• squad plugin uninstall <name> removes every file recorded in the lock and clears registration entries.
• Re-installing the same lock yields a byte-identical result (reproducibility test).
• No execution of plugin-supplied scripts at any point.

Acceptance

• Install + uninstall round-trip leaves the working tree byte-identical.
• Lock file format documented and stable for v0.1.
• Tests cover path source, git source, partial-failure rollback, and reproducibility.

Planning provenance: tamirdresher_microsoft/tamresearch1#2918.