feat(plugin): /claude-to-agents — convert CLAUDE.md to AGENTS.md for codex / gemini users

Cross-tool adoption. Codex, Gemini Code Assist, and any AI tool honouring
the AGENTS.md convention can now read the same instructions as Claude,
without the user maintaining two files by hand.

hooks/claude-to-agents.py (new, standalone, idempotent):
  - argparse: --mode={symlink,copy,inline-chain} (default symlink),
    --source (default CLAUDE.md), --output (default AGENTS.md), --force.
  - Symlink mode: ln -s CLAUDE.md AGENTS.md. Windows falls back to
    --copy with a stderr notice.
  - Copy mode: byte-for-byte snapshot via shutil.copyfile.
  - Inline-chain mode: depth-first walk of @path imports, recursive,
    cycle-safe (each file read at most once). Output flattens every
    chained sub-file under <!-- inlined from <rel> --> markers and
    strips two flavours of Claude-only scaffolding:
      • The @path import lines themselves (other tools don't resolve
        them).
      • Backlink quote-blocks ("> Parent context: ..." /
        "> Chained import: `@../CLAUDE.md`") that we emit on every
        sub-CLAUDE.md.
  - Safety: existing AGENTS.md (file or symlink) is renamed to
    AGENTS.md.backup.<UTC-ts> before overwrite. --force skips the
    backup. Microsecond timestamp precision so back-to-back writes
    don't collide.
  - Exit codes: 0 success, 1 user error (missing source / unknown
    mode), 2 filesystem error (symlink failure).

command/claude-to-agents.md (new slash command):
  - allowed-tools: Read, Write, Glob, Bash(python3:*), Bash(ls:*),
    Bash(test:*), Bash(readlink:*).
  - disallowedTools: WebFetch, WebSearch (no exfiltration vector).
  - argument-hint and when_to_use surface the three modes.
  - Body specifies a heuristic: default to --symlink for single-file
    projects, recommend --inline-chain when find . -name CLAUDE.md
    returns more than one. Documents per-mode verification commands.

.claude-plugin/plugin.json:
  Registers ./command/claude-to-agents.md alongside the existing two.

install.sh + install.ps1:
  Banner and uninstall sections list the new command. The command
  install loop already iterates command/*.md so the file itself is
  copied automatically.

README.md:
  /claude-to-agents added to "What's Included" under Slash commands
  with a one-paragraph description covering all three modes.
  Quick Stats: "2 slash commands" -> "3 slash commands".

CHANGELOG.md:
  New "wave 5" entry under [Unreleased].

Verified (7/7 integration checks):
  - Plugin manifest registers the command; all 9 referenced paths
    resolve on disk.
  - Slash command frontmatter has all required fields including
    WebFetch in disallowedTools.
  - Script has executable bit set.
  - Both install scripts list claude-to-agents.md.
  - install.sh passes bash -n.
  - End-to-end against a synthetic chained CLAUDE.md tree with a
    deliberate back-import cycle: symlink mode creates valid symlink;
    copy mode produces bytewise-identical AGENTS.md and backs up the
    prior symlink (1 backup); inline-chain mode inlines both
    sub-files, strips backlinks AND chain-import lines, handles the
    cycle (each file read once), backs up the prior file (2 backups
    total).
  - Missing source returns rc=1 with stderr message.

CHANGELOG.md

@@ -9,6 +9,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added (wave 5 — CLAUDE.md → AGENTS.md conversion for Codex / Gemini)
+
+Cross-tool adoption: every project using ClaudeForge can now share its instructions with non-Claude tools (OpenAI Codex, Gemini Code Assist, anything honouring the AGENTS.md convention) without maintaining two files.
+
+- **`/claude-to-agents`** slash command (`command/claude-to-agents.md`) with three modes:
+  - `--symlink` (default on macOS / Linux) — `AGENTS.md` becomes a symlink to `CLAUDE.md`. One source of truth; edits propagate instantly. Windows falls back to `--copy` automatically with a stderr notice.
+  - `--copy` — byte-for-byte snapshot. Use when the user wants to fork the instructions or when their VCS/build pipeline doesn't follow symlinks.
+  - `--inline-chain` — recursively walks every `@path/.../CLAUDE.md` import and writes a single flat AGENTS.md with all sub-file content inlined. **Recommended for Codex / Gemini in modular projects** because those tools don't auto-resolve `@`-imports.
+- **`hooks/claude-to-agents.py`** — standalone, idempotent script. Backs up an existing `AGENTS.md` to `AGENTS.md.backup.<UTC-timestamp>` before overwrite (unless `--force`). Strips Claude-only scaffolding (backlink lines, `@`-import lines) from `--inline-chain` output. Cycle-safe (each file read at most once).
+- Plugin manifest registers the new command. Both installers list it in their banner and uninstall instructions.
+
 ### Added (wave 4 — forked task-style audit skills)
 
 Three new task-style skills under `skill/`, each using Anthropic's `context: fork` + `agent: Explore` so they run in an isolated subagent context (no caller chat history, ≤500-token summary back to the main session):