fix(pstack): drop agent-selection language from slash-only skill descriptions

[deanrie]

Summary

Fixes #367.

Four pstack skills set disable-model-invocation: true (slash-only) but their description still reads as agent-selection guidance (Use only when the user explicitly asks..., Use when reading or editing..., Use when the user says reflect). That phrasing only makes sense for model auto-invocation; on a slash-only skill it lands in context on every /tdd and tells the model when to pick a skill it cannot pick. This rewrites those four descriptions as a short capability summary plus the slash trigger, and leaves the flag in place.

• tdd: describe the workflow and when it applies; drop Use only when the user...
• typescript-best-practices: capability summary; keep paths and the flag
• make-bot-ui: capability summary plus /make-bot-ui
• reflect: Run it with /reflect
• plugin.json: 0.15.15 -> 0.15.16

Same scope as #392, which has been conflicting with main since Sep 25 with no update; this is the same change rebased on the current tree. unslop is covered separately by #514.

Test plan

• npm install --no-save ajv ajv-formats && node scripts/validate-plugins.mjs: All plugins validated successfully
• git diff --check clean
• disable-model-invocation: true still present on all four skills