refactor(plugin): collapse three host bundles into one shared plugin/ tree

[ScriptedAlchemy]

Stacked on #249 (base: feat/adoption-contract-injection). Merge last in the plugin stack.

What

Replaces the three duplicated host bundles (claude-plugin/, codex-plugin/, cursor-plugin/) — kept in lockstep by copy + parity tests — with a single plugin/ tree and per-host manifest dirs, the superpowers-style single-source/multi-manifest model.

The overriding invariant: deployed trees do not change

Captured golden sha256 manifests of all three hosts' deployed trees from the pre-refactor binary, then re-installed each host post-refactor: all three deploy byte-for-byte identically (paths + content hashes). This refactor changes only where files live in the repo and how they're embedded — not a single byte of what any host actually installs.

Layout

• plugin/skills/ — the 17 byte-identical canonical skills + 13 canonical-form workflow skills
• plugin/overlays/cursor/skills/ — the 13 Cursor disable-model-invocation dispatcher skills (kept as a Cursor overlay; the dispatcher→.cursor/commands/ migration is a deliberate follow-on)
• plugin/{commands,rules,agents}/ + plugin/overlays/cursor/agents/ (Cursor-form agents differ from Claude-form)
• plugin/hooks/hooks-{claude,cursor,codex}.json, plugin/.{claude,cursor,codex}-plugin/ manifests (contents unchanged so deployed manifests stay identical), plugin/.mcp.json + plugin/mcp-cursor.json, per-host READMEs
• src/agents/plugin_bundle.rs — PluginFile { relative (deploy path), contents }; CANONICAL_PLUGIN_FILES ∪ {CLAUDE,CURSOR,CODEX}_MANIFEST_FILES via {host}_files(). The deploy relative differs from the repo source path where needed (e.g. cursor deploys hooks/hooks.json from plugin/hooks/hooks-cursor.json) — that indirection is what keeps deployed trees byte-identical.
• The three installers now consume plugin_bundle::{host}_files(); the three *_EMBEDDED_PLUGIN_FILES tables are gone. steering.rs repointed.

Tests

• Deleted the now-moot cross-copy parity machinery (plugin_bundle_sync_test, codex *_match_the_cursor_source_for_parity / ships_exactly_the_model_invocable, claude byte-identity-to-codex) — with one source copy there's nothing to sync.
• Rewrote the three *_embedded_file_list_covers_the_whole_source_bundle as composed-view checks; retargeted the manifest/config/skill-contract/skill-lint/coverage/update-plugin tests to the plugin/ tree (staging composed per-host deploy layouts in temp dirs where source≠deploy).
• fmt/clippy/build clean; full suite green (lib 773, agent_suite 413, mcp 307, +others). Per-host install smoke-test proves deployed trees unchanged.

Deferred (follow-on pass, task #11 remainder)

Recursive-walk embedding for skill support-files; cursor dispatcher→.cursor/commands/; memory-skill merge + message_search lane split; unified shared_skill_contract_test.

🤖 Generated with Claude Code

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