feat(hooks): agent-fork-logger PreToolUse:Agent

Advisory hook (RULE 0.12). Reads Agent tool_input JSON from stdin, hashes
the prompt (SHA-256 first 16), derives agent id and branch per isolation
mode, emits kei-ledger fork row.

NEVER blocks: every exit path is exit 0. Missing jq / kei-ledger / git
= silent no-op. Advisory because isolation=false trivial agents are
expected to slip through (RULE 0.12 Exceptions 1-2).

Constructor Pattern: 51 LOC, POSIX sh, chmod +x.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hooks/agent-fork-logger.sh

@@ -0,0 +1,51 @@
+#!/bin/sh
+# agent-fork-logger.sh — PreToolUse:Agent advisory hook (RULE 0.12).
+#
+# Reads the Agent tool invocation JSON from stdin and, if kei-ledger is on
+# PATH, logs a fork row so the orchestrator can later validate the bundle.
+# NEVER blocks: every exit path is `exit 0`. Hard-dependency absent (no jq,
+# no kei-ledger, no git) → silent no-op.
+
+command -v jq >/dev/null 2>&1 || exit 0
+
+set -eu
+
+input="$(cat)"
+
+# Extract subagent_type / prompt / isolation from the Agent tool_input.
+subagent=$(printf '%s' "$input" | jq -r '.tool_input.subagent_type // empty' 2>/dev/null || true)
+prompt=$(printf '%s' "$input" | jq -r '.tool_input.prompt // empty' 2>/dev/null || true)
+isolation=$(printf '%s' "$input" | jq -r '.tool_input.isolation // empty' 2>/dev/null || true)
+
+# No subagent name → not an agent call we know how to track.
+if [ -z "$subagent" ]; then
+    exit 0
+fi
+
+# Spec fingerprint = first 16 hex chars of SHA-256(prompt).
+spec_sha=$(printf '%s' "$prompt" | shasum -a 256 2>/dev/null | cut -c1-16)
+spec_sha=${spec_sha:-unknown}
+
+ts=$(date +%s)
+agent_id="${subagent}-${ts}"
+
+if [ "$isolation" = "worktree" ]; then
+    branch="agent/${subagent}-${ts}"
+else
+    branch="inline-${subagent}-${ts}"
+fi
+
+# Current git branch (if we're inside a repo). Non-fatal if git absent.
+current_branch=$(git rev-parse --abbrev-ref HEAD 2>/dev/null || echo none)
+
+# If kei-ledger is unavailable, bail silently — the hook is advisory only.
+command -v kei-ledger >/dev/null 2>&1 || exit 0
+
+# Emit the fork row. Always exit 0 even if kei-ledger returns non-zero:
+# the hook must never block an Agent invocation on ledger trouble.
+kei-ledger fork "$agent_id" "$branch" \
+    --parent "$current_branch" \
+    --spec-sha "$spec_sha" \
+    >/dev/null 2>&1 || true
+
+exit 0