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 Superseded in Phase 2 by the plugin-bundled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>

scripts/scribe_session_context.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# Scribe SessionStart hook (dogfood / pre-plugin).
+#
+# Curls the running Scribe dev instance for the operator's always-on rules +
+# active-project context and emits it as SessionStart `additionalContext` — the
+# push channel that lets Scribe surface reflexively (plan #755, Phase 1).
+#
+# FAIL-OPEN by design: any missing tool, missing config, network error, or
+# unreachable instance results in injecting nothing and exiting 0. A session
+# must NEVER be blocked because Scribe is down. Reads URL+token from the
+# project's .mcp.json so no secret lives in this tracked script.
+#
+# Superseded in Phase 2 by the same hook bundled in the Scribe plugin, where
+# URL+token come from plugin userConfig instead of .mcp.json.
+set -uo pipefail
+
+MCP_JSON="${CLAUDE_PROJECT_DIR:-.}/.mcp.json"
+command -v jq   >/dev/null 2>&1 || exit 0
+command -v curl >/dev/null 2>&1 || exit 0
+[ -f "$MCP_JSON" ] || exit 0
+
+# scribe-dev is the locally-wired instance (prod id 2 / dev id 3 per binding).
+url=$(jq -r '.mcpServers["scribe-dev"].url // empty' "$MCP_JSON" 2>/dev/null) || exit 0
+auth=$(jq -r '.mcpServers["scribe-dev"].headers.Authorization // empty' "$MCP_JSON" 2>/dev/null) || exit 0
+token=${auth#Bearer }
+[ -n "$url" ] && [ -n "$token" ] || exit 0
+
+base=${url%/mcp}
+body=$(curl -fsS --max-time 8 \
+  -H "Authorization: Bearer ${token}" \
+  "${base}/api/plugin/context?project_id=3" 2>/dev/null) || exit 0
+
+text=$(printf '%s' "$body" | jq -r '.context // empty' 2>/dev/null) || exit 0
+[ -n "$text" ] || exit 0
+
+jq -n --arg c "$text" \
+  '{hookSpecificOutput: {hookEventName: "SessionStart", additionalContext: $c}}'
+exit 0