fix(hooks): stop auto-injecting the meta-skill on SessionStart (#569)

The plugin's SessionStart hook injected using-agent-skills/SKILL.md as
additionalContext on every session. Claude Code already discovers and
routes skills from their descriptions, so the hook stacks a second
router on the native one — the pattern docs/getting-started.md warns
against ("That creates two routers for the same task"), measured at
~3.5k wasted cache-creation tokens per session in #569.

Delete hooks/hooks.json (the registration; the plugin now ships no
hooks) and keep hooks/session-start.sh as a standalone script for hosts
without native skill routing, matching the fix shape suggested in the
issue. Update the hook header, CONTRIBUTING.md, codex-setup.md, and
gemini-cli-setup.md to match.

CONTRIBUTING.md

@@ -81,7 +81,7 @@ We don't accept translations of the documentation (README, `docs/`) or of skills
 
 ## Testing Hooks
 
-The session-start hook (`hooks/session-start.sh`) injects the `using-agent-skills` meta-skill into every new Claude Code session. A regression test at `hooks/session-start-test.sh` validates the hook's JSON payload — both when `jq` is available and when it isn't.
+The session-start script (`hooks/session-start.sh`) injects the `using-agent-skills` meta-skill when wired into a host's `SessionStart` hook. The Claude Code plugin does not register it — Claude Code routes skills natively, and always-on injection would create two routers for the same task (see [docs/getting-started.md](docs/getting-started.md)); the script remains for hosts without native skill routing. A regression test at `hooks/session-start-test.sh` validates the script's JSON payload — both when `jq` is available and when it isn't.
 
 Run it before opening any PR that touches:
 

docs/codex-setup.md

@@ -32,4 +32,4 @@ After install, invoke a skill in Codex chat with `@` (e.g. `@spec-driven-develop
 - `.agents/plugins/marketplace.json` — marketplace entry declaring the repo root (`./`) as the plugin source.
 - `skills/<name>/SKILL.md` — unchanged. Codex and Claude Code share the same `name` + `description` frontmatter format, so one file serves both platforms.
 
-Slash commands in `.claude/commands/`, personas in `agents/`, and the lifecycle hook under `hooks/` stay Claude Code-specific. On Codex, invoke the underlying skill directly instead of the slash command (e.g. `@spec-driven-development` instead of `/spec`).
+Slash commands in `.claude/commands/` and personas in `agents/` stay Claude Code-specific. The `SessionStart` script under `hooks/` is a standalone helper for hosts without native skill routing and is not wired by either plugin. On Codex, invoke the underlying skill directly instead of the slash command (e.g. `@spec-driven-development` instead of `/spec`).

docs/gemini-cli-setup.md

@@ -93,7 +93,7 @@ To enable these, ensure you have the relevant MCP extensions installed in your G
 
 Gemini CLI supports session lifecycle hooks. You can use these to automatically inject context or run validation scripts at the start of a session.
 
-To replicate the `agent-skills` experience from other tools, you can configure a `SessionStart` hook that reminds you of the available skills or loads a meta-skill.
+To replicate the `agent-skills` experience from other tools, you can configure a `SessionStart` hook that reminds you of the available skills or loads a meta-skill. `hooks/session-start.sh` in this repo is a ready-made script for that: it prints the `using-agent-skills` meta-skill as a standard `SessionStart` JSON envelope on stdout.
 
 ### Explicit Context Loading
 

hooks/hooks.json

@@ -1,14 +0,0 @@
-{
-  "hooks": {
-    "SessionStart": [
-      {
-        "hooks": [
-          {
-            "type": "command",
-            "command": "SCRIPT=\"${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh\"; [ -f \"$SCRIPT\" ] || SCRIPT=\"${CLAUDE_PROJECT_DIR}/.claude/hooks/session-start.sh\"; [ -f \"$SCRIPT\" ]&& bash \"$SCRIPT\" || true"
-          }
-        ]
-      }
-    ]
-  }
-}

hooks/session-start.sh

@@ -1,6 +1,11 @@
 #!/bin/bash
 # agent-skills session start hook
-# Injects the using-agent-skills meta-skill into every new session
+# Injects the using-agent-skills meta-skill into a new session.
+#
+# Not wired by the plugin: hosts that already route skills from their
+# descriptions (Claude Code, Codex CLI) would run a second router on top of
+# the native one — see docs/getting-started.md. Wire this script into a
+# SessionStart hook only on hosts without native skill routing.
 #
 # Every output path must emit the standard SessionStart envelope
 #   {"hookSpecificOutput": {"hookEventName": "SessionStart", "additionalContext": "..."}}