feat(plugin): add /api/plugin/context push-channel endpoint + dogfood hook

Phase 1 of plan #755 (Scribe-as-plugin). Gives Scribe its own session-start
push channel so always-on rules + active-project context surface without being
asked — the gap behind 'I have to prompt for everything'.

- services/plugin_context.build_session_context: renders always-on rule titles
  grouped by topic (under the 10k additionalContext cap; full text stays one
  list_always_on_rules/get_rule call away) + optional project goal/open-task
  count + a recall/update-over-create reflex line. Capped at 9000 chars.
- routes/plugin GET /api/plugin/context (login_required already accepts Bearer
  fmcp_ keys; read scope suffices).
- tests: titles-not-statements, project scoping, length cap (pure mocks).
- scripts/scribe_session_context.sh: dogfood SessionStart hook, fail-open,
  reads url+token from .mcp.json. Superseded in Phase 2 by the plugin-bundled
  hook using userConfig.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_services_plugin_context.py

@@ -0,0 +1,69 @@
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+def _rule(rid, title, topic_id):
+    r = MagicMock()
+    r.id, r.title, r.topic_id = rid, title, topic_id
+    r.statement = "FULL STATEMENT SHOULD NOT BE INJECTED"
+    return r
+
+
+@pytest.mark.asyncio
+async def test_build_session_context_renders_titles_grouped_by_topic():
+    rules = [
+        _rule(1, "`dev` is home", 1),
+        _rule(2, "Release — never without explicit request", 1),
+        _rule(3, "No GitHub — Fabled-Git only", 2),
+    ]
+    with patch("scribe.services.plugin_context.rulebooks_svc.list_always_on_rules",
+               AsyncMock(return_value=rules)), \
+         patch("scribe.services.plugin_context._topic_titles",
+               AsyncMock(return_value={1: "git-workflow", 2: "fabled-git"})):
+        from scribe.services.plugin_context import build_session_context
+        out = await build_session_context(user_id=7, project_id=0)
+
+    ctx = out["context"]
+    assert out["rule_count"] == 3
+    assert out["project"] is None
+    # Titles present, grouped under topic headings
+    assert "### git-workflow" in ctx
+    assert "### fabled-git" in ctx
+    assert "- [1] `dev` is home" in ctx
+    assert "- [3] No GitHub — Fabled-Git only" in ctx
+    # Full statements must NOT be dumped (push channel injects titles only)
+    assert "FULL STATEMENT SHOULD NOT BE INJECTED" not in ctx
+
+
+@pytest.mark.asyncio
+async def test_build_session_context_includes_project_when_scoped():
+    project = MagicMock(id=2, title="FabledScribe", goal="ship it")
+    with patch("scribe.services.plugin_context.rulebooks_svc.list_always_on_rules",
+               AsyncMock(return_value=[_rule(1, "rule", 1)])), \
+         patch("scribe.services.plugin_context._topic_titles",
+               AsyncMock(return_value={1: "git-workflow"})), \
+         patch("scribe.services.plugin_context.projects_svc.get_project",
+               AsyncMock(return_value=project)), \
+         patch("scribe.services.plugin_context.notes_svc.list_notes",
+               AsyncMock(return_value=([], 4))):
+        from scribe.services.plugin_context import build_session_context
+        out = await build_session_context(user_id=7, project_id=2)
+
+    assert out["project"] == {"id": 2, "title": "FabledScribe"}
+    assert "## Active project: FabledScribe (id 2)" in out["context"]
+    assert "Open todo tasks: 4" in out["context"]
+
+
+@pytest.mark.asyncio
+async def test_build_session_context_caps_length():
+    many = [_rule(i, "x" * 200, 1) for i in range(200)]
+    with patch("scribe.services.plugin_context.rulebooks_svc.list_always_on_rules",
+               AsyncMock(return_value=many)), \
+         patch("scribe.services.plugin_context._topic_titles",
+               AsyncMock(return_value={1: "git-workflow"})):
+        from scribe.services.plugin_context import build_session_context
+        out = await build_session_context(user_id=7)
+
+    assert len(out["context"]) <= 9000 + 60  # cap + truncation note
+    assert "truncated" in out["context"]