feat: /plan graduates from bundled skill to built-in command on every surface

The bundled plan skill's auto-generated slash command fell off the capped
Telegram/Discord command menus for most installs (skills are the only tier
trimmed at the platform caps, alphabetically — 'plan' sat past the cutoff at
index 57 of 82 bundled skills). Converting it to a first-class CommandDef
gives it a guaranteed core-tier menu slot on every platform.

- agent/plan_prompt.py: build_plan_prompt() — plan-mode rules + authoring
  craft distilled from the retired skill; prompt-injection pattern like
  /learn and /init (no engine, no model-tool footprint, cache-safe).
- CLI: _handle_plan_command mixin handler (pending-input injection).
- Gateway: /plan branch rewrites event.text and falls through (role
  alternation preserved).
- TUI: command.dispatch branch ('plan' was already in
  _PENDING_INPUT_COMMANDS).
- Removed skills/software-development/plan/ + docs pages (EN + zh-Hans),
  catalog rows, sidebar entry, related_skills references.
- PROTECTED_BUILTIN_SKILLS is now empty (mechanism kept); dependent
  curator/usage tests moved to monkeypatched sentinels.

Salvages #67292 by @webtecnica (credit: first /plan command submission,
issue #67264); reworked from inline planning prompt to the prompt-injection
pattern with workspace-saved plans. Closes #67264, closes #36821 (empty
/plan infers task from conversation context).

optional-skills/software-development/grill-me/SKILL.md

@@ -8,7 +8,7 @@ platforms: [linux, macos, windows]
 metadata:
   hermes:
     tags: [planning, adversarial, interview, decision-tree, pre-implementation, review, alignment]
-    related_skills: [plan, requesting-code-review, subagent-driven-development, test-driven-development]
+    related_skills: [requesting-code-review, subagent-driven-development, test-driven-development]
 ---
 
 # Grill Me

optional-skills/software-development/subagent-driven-development/SKILL.md

@@ -8,7 +8,7 @@ platforms: [linux, macos, windows]
 metadata:
   hermes:
     tags: [delegation, subagent, implementation, workflow, parallel]
-    related_skills: [plan, requesting-code-review, test-driven-development]
+    related_skills: [requesting-code-review, test-driven-development]
 ---
 
 # Subagent-Driven Development