feat(prism-facet): add 'always' router + Hangman-Lab pcexec convention prompt

The 'always' router resolves to the constant key "always" for every
agent — pair with a rule like 'always:always → some-prompt.md' to
inject a prompt fragment unconditionally (no ego/role/position
lookup needed).

Bundle a site-specific prompt 'pcexec-convention.md' that tells every
agent: Hangman-Lab keeps site binaries at ~/.openclaw/bin (hf,
secret-mgr, ego-mgr, fabric-register, pcguard, lock-mgr, tea) — not
symlinked to /usr/local/bin — so they MUST be invoked via the pcexec
tool, not the codex built-in shell. Without this, agents would call
those CLIs directly and get 'command not found' (observed during the
2026-05-23 hf-wakeup runs on prod t2).

Register the binding in rules.json so it loads at gateway startup.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/rules.json

@@ -0,0 +1,3 @@
+{
+  "always:always": "/root/.openclaw/plugins/prism-facet/prompts/pcexec-convention.md"
+}