fix(pstack): slash-only skill descriptions and skill names that match their directories

[deanrie]

Summary

Frontmatter fixes for pstack skills in one PR, consolidated from #519 and #596 (both closed in favour of this one; commits carried unchanged, one each). Docs-only; no plugin.json bump, following the pattern where pstack bumps land as separate chore PRs (#581, #584, #594).

1. fix(pstack): drop agent-selection language from slash-only skill descriptions (was fix(pstack): drop agent-selection language from slash-only skill descriptions #519, fixes pstack: disable-model-invocation skills still have "Use only when user ..." agent instructions #367). tdd, reflect, make-bot-ui and typescript-best-practices carry disable-model-invocation: true but their descriptions still read as agent-routing guidance (Use only when the user explicitly asks..., Use when reading or editing any .ts file). On a slash-only skill that text lands in context on every /tdd and tells the model when to pick a skill it cannot pick. Rewritten as a capability summary plus the slash trigger; flags and paths untouched. The 0.15.16 bump that fix(pstack): drop agent-selection language from slash-only skill descriptions #519 carried is dropped here (main is on 0.15.24).
2. fix(pstack): make skill names match their directories (poteto-mode, make-bot-ui) (was fix(pstack): make skill names match their directories (poteto-mode, make-bot-ui) #596). The skills docs require name to be lowercase/hyphens and equal to the parent folder; these two had Poteto Mode and Make Bot UI. Now poteto-mode and make-bot-ui.

Verification

• node scripts/validate-plugins.mjs on this branch: All plugins validated successfully.
• Descriptions on current main are unchanged since docs(pstack): cut 19 more instructions Opus 5.5 does not need #419 (09-23), so the rewrite in commit 1 still applies cleanly.

Fixes #367. Related: #269 / #600 (same name rule for the X plugins), #618 (validator starts warning on these mismatches).