… G2c (closes #61)

Three new devagentic-mutations MCP tools, shipped together because
``patch_artifact`` is unworkable without ``preview_patch`` outside
dev-scoped contexts (the confirm_token gate makes them a unit).

## Tools registered

| Tool | Wraps | Notes |
|---|---|---|
| ``read_artifact(path, ctx_id?, strategy?="raw")`` | ``readArtifact`` mutation | 32K cap, or 256K when ctx_id is dev-scoped |
| ``preview_patch(path, find, replace)`` | ``previewPatch`` query field | Returns ``{confirm_token, diff, ...}``; read-only |
| ``patch_artifact(path, find, replace, confirm_token?, ctx_id?)`` | ``patchArtifact`` mutation | Requires ``confirm_token`` outside dev-scoped/sandbox-backend ctx |

#61's scope was nominally 2 tools (read + patch). Reality: the
confirm_token gate makes ``patch_artifact`` unusable without
``preview_patch`` — so the three are shipped as the natural unit.
``previewPatch`` is a strawberry field on Query (not Mutation);
client + wrapper handle that distinction transparently.

## Devagentic-side resolver verification

Schemas read directly from the devagentic api.py via gh api:
- ``readArtifact(path, ctx_id?, strategy?, strategy_args?) -> JSON``
- ``previewPatch(path, find, replace) -> JSON``
- ``patchArtifact(path, find, replace, confirm_token?, ctx_id?) -> JSON``

The arg-name camelCase conversion (ctx_id → ctxId, confirm_token →
confirmToken) matches strawberry's default snake-to-camel behavior.

## Tests

- 12 new client-level tests (4 per tool) in
  ``tests/plugins/devagentic-mutations/test_client.py``: empty-arg
  fail-soft, happy-path round-trip with arg threading, strategy /
  ctx_id / confirm_token threading, in-band errors, non-dict
  response fail-soft.
- Contract test in ``test_mcp_defensive_registrars.py`` extended:
  ``read_artifact`` / ``preview_patch`` / ``patch_artifact`` added
  to the required-G2 tool set.
- 190 total green across affected suites (mutations + MCP + canvas
  + docs + subset-filter + autowire). No regression.

## Tracking
- Closes #61 (G2c).
- G2d (#62 fetchUrl) remains open as the last G2 sub-issue.