Add native Gstack plan writing skill

A native plan writing skill would ensure a more robust and repeatable plan output, and eliminate the brittleness of dropping out of the Gstack flow to write the plan (currently using Claude's Plan Mode).

Instead have a formal hand-off from Office Hours / CEO / Eng plan review / Design plan review to a dedicated "Write Plan" skill, and maybe even then to a dedicated "Implementation" skill.

That creates a dedicated pipeline from Office Hours all the way through to /Review, /QA, and /Ship.

This all assumes Plan mode is supposed to be turned on from the beginning of the pipeline, as indicated in the Readme.

See below for conflicts:

> I’m realizing plan mode has a constraint—I can only modify the plan file itself, not write to external locations like ~/.gstack/. Since the plan file already contains all the necessary information about scope decisions and verdicts, I’ll skip the CEO plan file write and note this limitation in the review instead. I confirmed that Bash commands can write to ~/.gstack/ since they bypass the file-edit allowlist and work through bash permissions directly. Let me run the review log command, offer a brief outside perspective, and then exit plan mode — that’s what the user is really after.

> The user invoked /office-hours explicitly while in plan mode, which creates a conflict since office-hours is designed for exploring “should we build this” but they’ve already decided what to build and need architectural guidance. Per the skill rules, /office-hours takes precedence in plan mode, but the user’s actual need is implementation planning rather than design exploration.
>
> I should honor both constraints: run a light office-hours exploration (skipping the questioning phase since they have a clear vision, but still doing premise challenge and alternatives), then produce a concrete implementation plan that addresses their architectural concerns about scoping and DRI workflow integration.