fix(hooks): emit the standard SessionStart envelope (#465)

The session-start hook emitted {"priority", "message"} JSON on all three
of its output paths. Codex CLI validates SessionStart hook output and
fails the hook on that shape ('hook returned invalid session start JSON
output', #465); the documented contract for both Codex and Claude Code is

  {"hookSpecificOutput": {"hookEventName": "SessionStart",
                          "additionalContext": "..."}}

Switch all three paths (meta-skill injection, jq-missing fallback,
meta-skill-missing fallback) to that envelope. Verified each path emits
valid JSON of the new shape: happy path via jq shape assertion, jq-missing
via PATH='' run, meta-skill-missing via a copy outside the repo.

hooks/session-start.sh

@@ -1,13 +1,17 @@
 #!/bin/bash
 # agent-skills session start hook
 # Injects the using-agent-skills meta-skill into every new session
+#
+# Every output path must emit the standard SessionStart envelope
+#   {"hookSpecificOutput": {"hookEventName": "SessionStart", "additionalContext": "..."}}
+# Hosts that validate hook output (Codex CLI, Claude Code) reject other shapes.
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 SKILLS_DIR="$(dirname "$SCRIPT_DIR")/skills"
 META_SKILL="$SKILLS_DIR/using-agent-skills/SKILL.md"
 
 if ! command -v jq >/dev/null 2>&1; then
-  echo '{"priority": "INFO", "message": "agent-skills: jq is required for the session-start hook but was not found on PATH. Install jq (e.g. `brew install jq` or `apt-get install jq`) to enable meta-skill injection. Skills remain available individually."}'
+  echo '{"hookSpecificOutput": {"hookEventName": "SessionStart", "additionalContext": "agent-skills: jq is required for the session-start hook but was not found on PATH. Install jq (e.g. `brew install jq` or `apt-get install jq`) to enable meta-skill injection. Skills remain available individually."}}'
   exit 0
 fi
 
@@ -15,10 +19,10 @@ if [ -f "$META_SKILL" ]; then
   CONTENT=$(cat "$META_SKILL")
   # Use jq to properly escape and construct valid JSON
   jq -cn \
-    --arg message "agent-skills loaded. Use the skill discovery flowchart to find the right skill for your task.
+    --arg context "agent-skills loaded. Use the skill discovery flowchart to find the right skill for your task.
 
 $CONTENT" \
-    '{priority: "IMPORTANT", message: $message}'
+    '{hookSpecificOutput: {hookEventName: "SessionStart", additionalContext: $context}}'
 else
-  echo '{"priority": "INFO", "message": "agent-skills: using-agent-skills meta-skill not found. Skills may still be available individually."}'
+  echo '{"hookSpecificOutput": {"hookEventName": "SessionStart", "additionalContext": "agent-skills: using-agent-skills meta-skill not found. Skills may still be available individually."}}'
 fi