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>