Add an agent adapter

An adapter is declarative metadata in src/synapse/cli/adapters/registry.py interpreted by shared machinery. Most of the work is not code — it is extending literal test tables and docs that are deliberately independent of the registry. The classic failure mode is a missed surface, so treat this as a checklist and finish every section.

1. Registry entry

Add one AgentAdapter(...) to ADAPTERS. The model lives in src/synapse/cli/adapters/model.py: McpTarget (format, container shape, key_path, payload_style, extra_fields, project/user PathSpecs), InstructionTarget (InstructionMode.BLOCK splices a heading-anchored section into a shared file; OWNED writes a dedicated file, optionally with frontmatter), PathSpec env overrides (env_var replaces env_prefix; env_var_full replaces the whole path), and optional HookTarget.

Copy the closest exemplar rather than composing from scratch:

• claude-code — full surface (project+user MCP, both instructions, both skills, hook)
• copilot-vscode — minimal project-only (no user MCP, no globals, no hook)
• hermes — global-only, default_scope="user", YAML
• codex — TOML, skill_files override (extra agents/openai.yaml), warn_legacy_user_config
• crush — HookShape.FLAT, hook settings file is the MCP config file itself
• opencode — PayloadStyle.COMMAND_LIST plus document_defaults ($schema)

paths.py, render.py, instructions.py, skills.py, and marker_blocks.py are adapter-agnostic — a new adapter should need no changes there. The instruction snippets are the shared packaged templates in src/synapse/adapters/; the only per-adapter difference is the synapse doctor --agent <id> line (test_agent_snippets_differ_only_in_doctor_line enforces this).

2. Hooked adapters only

A hook needs both the registry HookTarget and a new HookCodec in src/synapse/cli/hooks/codecs.py (CODECS is keyed by the HookTarget.codec string). Codecs must be fail-open: always exit 0, never block the agent's tool call, and preserve any allow/permission decision the agent expects (see qwen-pre-bash for an explicit permissionDecision: allow). Then update the literal rosters:

• tests/cli/test_adapter_cli.py — expected = ["claude-code", "crush", "qwen"] hook list
• tests/cli/test_hooks.py — add a SHELL_TOOLS entry for the new codec

3. Literal matrix tests

tests/cli/test_adapter_matrix.py states its own design: expectations are written out literally "so a registry typo cannot make these tests agree with themselves". Never derive these values from the registry — type them from the target agent's documentation:

• EXPECTED_PATHS[id] — the 6-tuple (project mcp, user mcp, project instructions, global instructions, project skill, global skill), None for unsupported surfaces
• EXPECTED_ENTRIES[id] — the exact serialized MCP entry the agent's config should contain
• _SEEDS — only if the adapter introduces a genuinely new (format, shape) combination

test_matrix_covers_every_adapter gates completeness against ADAPTERS and adapter_choices(), so forgetting a table fails loudly.

4. Other test files to check

• tests/cli/test_adapters.py — shared-file groups are hardcoded (the AGENTS.md sharers tuple, test_both_copilot_adapters_keep_a_single_block); extend them if the new adapter shares an instruction file or skills directory.
• Grep tests/cli/test_setup.py, test_global_install.py, test_doctor.py, test_init_status.py, test_managed_workflow.py for hardcoded agent ids before finishing.
• tests/cli/conftest.py fixtures derive from the registry; usually no edit unless a new env-override kind is introduced.

5. Shared-skill edge

Several adapters share one skills directory (.agents/skills/, .github/skills/). The keep-until-last-uninstall behavior is computed live by agents_sharing_skill in src/synapse/cli/installer.py — same resolved skill path and the other adapter's MCP config still contains a Synapse entry. A new sharing adapter needs no extra code, but must be named in the shared-file prose of docs/installation.md and covered by the shared-skill tests.

6. Docs

docs/installation.md:

• "Global files" table (agent, id, user MCP path + hook file, global instructions/skill, env override) and "Project files" table
• shared-file/shared-skill prose
• one "Per-agent notes" bullet explaining the adapter's quirk
• "Hooks" table if hooked — only context-injecting, allow-preserving hooks qualify
• remove the agent's row from "Investigated and deferred" if present

Add a CHANGELOG.md entry under ## [Unreleased]. README.md links the support matrix rather than duplicating it; check its agent-count phrasing.

7. Gate

uv run ruff check && uv run mypy && uv run pytest -q tests/cli