#20)

Two changes in this PR — one a defect fix, one the rest of #12's
proposed scope.

Schema fix (defect in #19 MVP):
- The actual schema is searchDocs(query, k), not (query, limit, tag).
  My PR #19 shipped invalid GraphQL that the server would reject.
  Confirmed via introspection of devagentic's live schema.
- Route /doc search differently: with --tag, hit Query.docs(tags:[t])
  and filter client-side; without --tag, hit Query.searchDocs(query, k).
- Doc shape: drop `score` (not on Doc); add `source`, `ts`.

/fork surface (#12 follow-up):
- /fork open <parent_id> [--goal "..."]  wraps forkContext, sets a
  marker at $HERMES_HOME/docs-fork-active (sibling to canvas-active).
- /fork close clears the marker.
- /fork show prints active fork tags + annotations via context(id).
- /fork pin <doc_id> appends a pinned-doc annotation via decorateContext.
- /fork render exposes renderContext(ctxId) directly — useful for
  debugging the preamble injection.

pre_llm_call hook:
- When a fork is active, calls renderContext(ctxId) and returns
  {context: header + rendered} for the host to append to the user
  message. Capped at 8000 chars hermes-side; devagentic does its
  own bounding upstream.
- Returns None (turn proceeds without preamble) on any failure
  path: no marker, render returns None, render returns "".

Tests: 45 passed (43 prior + 2 net new after schema-fix rewrites).
The fixture now eagerly pre-loads all three submodules so
monkeypatches on plugin_pkg.client reach preamble's docs_client
reference.

Closes #12 in full.

Mirrors the canvas-tools pattern (#56) for the just-shipped
devagentic-docs plugin (#12 / PRs #19+#20+#23). MCP-aware clients
(Claude Code, Cursor, Codex) can now writeDoc / searchDocs /
forkContext / decorateContext / renderContext against any
devagentic instance hermes is configured for.

New tools (7):
- doc_search(query, limit, tag)  → search_docs
- doc_write(content, tags, source) → write_doc
- doc_show(doc_id)               → get_doc
- fork_open(parent_id, goal, tags) → fork_context (auto-pins
                                     parent + threads goal)
- fork_decorate(ctx_id, key, value, weight) → decorate_context
- fork_get(ctx_id)               → get_context
- fork_render(ctx_id)            → render_context (envelope:
                                   {ctx_id, rendered})

Loader pattern: _resolve_docs_client() mirrors
_resolve_canvas_client at line 880 — file-path import of
plugins/devagentic-docs/client.py since the hyphenated dir
isn't a Python package.

Failure semantics: every tool returns {"error": "<msg>"} JSON.
The plugin's last_error_text() pattern (introduced in #15)
threads through via a _reason() helper, so federated agents
see the same actionable hints CLI users get — e.g. on the
canonical devbox deployment (where /graphql isn't exposed,
see #21), they'd see "not found at <url>/graphql ..." instead
of an opaque None.

Out of scope per #24: fork_close + fork_pin depend on the
session-local $HERMES_HOME/docs-fork-active marker, which
doesn't translate to MCP's stateless tool model. fork_open
auto-tags with source:hermes-mcp so MCP-authored forks are
distinguishable from CLI-authored ones (source:hermes-cli).

Tests: 22 new (tests/test_mcp_docs.py) covering registration,
plugin-missing fallback, arg routing, last_error surfacing for
the #21 case, and the real-import smoke. All passing alongside
canvas MCP suite (14 existing) and the broader devagentic-
adjacent test set (159 total).

Closes #24.