v1.19.0

## Highlights

### `engram setup --protocol=<slim|full>` — opt-in slim SessionStart hooks

Claude Code sessions receive the Engram memory protocol twice: once from the SessionStart hook's static prose and once from the MCP server instructions (served since v1.4.0). This release lets tooling opt a slug into a **slim** hook that drops the duplicated static prose (~3.6KB / ~900 tokens per session) while keeping everything dynamic and load-bearing:

- The session CONTEXT block (recent sessions, prompts, pinned and recent observations).
- The post-compaction recovery header and numbered steps — always emitted, in every mode.

### How it works

- `engram setup <slug> --protocol=<slim|full>` persists a per-slug mode atomically in `protocol-mode.json` under the data dir (`ENGRAM_DATA_DIR` honored end to end). Corrupted files are never overwritten and other slugs are never dropped.
- A new `engram protocol-mode <slug>` subcommand is the runtime read path for the version-pinned plugin hooks: it prints `slim` only when the persisted mode is slim **and** the binary meets the v1.4.0 instructions-channel floor — every failure path prints `full`. It is excluded from the startup update check, so hooks never pay a network call.
- `engram setup --help` now lists flags (including `--protocol`) and never reads stdin — external tools can probe capability safely with stdin detached and a timeout.
- `cmdSetup` argument handling is now a two-pass, order-independent classifier: `--protocol` is honored wherever it appears, unknown flags still fall back to the interactive menu, and setup never fails because of the flag.

### Compatibility

Fully backward compatible. No flag → today's behavior everywhere. Old hooks + new binary and new hooks + old binary both degrade safely to the full protocol text. Downstream consumers (gentle-ai) detect the flag automatically via `setup --help` and start forwarding it — no coordination needed beyond upgrading.

## Changelog

- feat(setup): add `--protocol` flag and `protocol-mode` runtime read path (#582, closes #581)

Verification: 25+ new Go tests (strict TDD), race-detector clean, live-binary probe validated, and a 16-entry adversarial review ledger archived with the change (`openspec/changes/archive/2026-07-08-setup-protocol-flag/`).