feat(plugin): two-tier SessionStart hook — static keyless floor + dynamic enrichment

The SessionStart push channel was single-tier: it curled /api/plugin/context
with a Bearer token and, on any failure (missing/unexported token, network
error, missing curl), injected nothing and exited 0 — silently. A known
upstream Claude Code gap (sensitive userConfig not reliably exported to hook
subprocesses) trips this routinely, so a fresh session gets no signal to reach
for Scribe and falls back to local file-memory (root cause of unlogged work on
remote/rc sessions).

Split into two tiers:
- Tier 1 (static, keyless, networkless, always fires): inject bundled
  scribe_static_context.md — the load-bearing behavioral mandate. Cannot be
  suppressed by the upstream key bug.
- Tier 2 (dynamic, best-effort, fails open): existing curl for live rules +
  active-project context, appended below the static block. Lights up as
  enrichment once the key reaches the hook.

Only jq is now required (JSON envelope); curl/token gate the dynamic tier only.
Bump plugin 0.1.5 -> 0.1.6 so clients pick up the change.

Refs milestone 55; task 809; decision note 810.

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

plugin/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "scribe",
   "description": "Scribe second brain for Claude Code: MCP tools over your notes/tasks/projects/rules, a session-start push channel that surfaces your always-on rules + active-project context, and a set of universal process-skills (brainstorm, debug, TDD, plan, verify). Replaces superpowers + file-memory with one app-backed plugin.",
-  "version": "0.1.5",
+  "version": "0.1.6",
   "author": { "name": "Bryan Van Deusen" },
   "mcpServers": {
     "scribe": {