feat: add /plan command (#1372)

* feat: add /plan command

* refactor: back /plan with bundled skill

* docs: document /plan skill

skills/software-development/plan/SKILL.md

@@ -0,0 +1,55 @@
+---
+name: plan
+description: Plan mode for Hermes — inspect context, write a markdown plan, save it under $HERMES_HOME/plans, and do not execute the work.
+version: 1.0.0
+author: Hermes Agent
+license: MIT
+metadata:
+  hermes:
+    tags: [planning, plan-mode, implementation, workflow]
+    related_skills: [writing-plans, subagent-driven-development]
+---
+
+# Plan Mode
+
+Use this skill when the user wants a plan instead of execution.
+
+## Core behavior
+
+For this turn, you are planning only.
+
+- Do not implement code.
+- Do not edit project files except the plan markdown file.
+- Do not run mutating terminal commands, commit, push, or perform external actions.
+- You may inspect the repo or other context with read-only commands/tools when needed.
+- Your deliverable is a markdown plan saved to `$HERMES_HOME/plans`.
+
+## Output requirements
+
+Write a markdown plan that is concrete and actionable.
+
+Include, when relevant:
+- Goal
+- Current context / assumptions
+- Proposed approach
+- Step-by-step plan
+- Files likely to change
+- Tests / validation
+- Risks, tradeoffs, and open questions
+
+If the task is code-related, include exact file paths, likely test targets, and verification steps.
+
+## Save location
+
+Save the plan with `write_file` under:
+- `$HERMES_HOME/plans/YYYY-MM-DD_HHMMSS-<slug>.md`
+
+If the runtime provides a specific target path, use that exact path.
+If not, create a sensible timestamped filename yourself.
+
+## Interaction style
+
+- If the request is clear enough, write the plan directly.
+- If no explicit instruction accompanies `/plan`, infer the task from the current conversation context.
+- If it is genuinely underspecified, ask a brief clarifying question instead of guessing.
+- After saving the plan, reply briefly with what you planned and the saved path.