#571 refactor(pstack): cut unneeded sections from seven skills

Summary

Removes sections from seven pstack skills where the agent does the same work without the text: restated rules, generic search steps, evidence checklists, and return-format blocks. The core rule of each skill stays.

Across the 11 touched Markdown files, size drops from 32,508 to 18,711 bytes (-42.4%) and from 5,028 to 2,902 words (-42.3%, counted with wc -w).

Changes

• principle-encode-lessons-in-structure: keeps the judgment fallback and the corollary. Drops the intro, the pattern steps, "Pick the strongest mechanism", the routing paragraph, and the anti-patterns.
• principle-exhaust-the-design-space: keeps the product-design line. The description still carries the rule (build 2-3 prototypes and compare). Drops the intro, "The rule", and the when / when-not lists.
• principle-guard-the-context-window: drops the intro and the "Isolate large payloads" and "Keep frequently used content inline" bullets.
• principle-laziness-protocol: keeps "Consolidate decisions" and "Minimize the diff". Drops the intro, "Prefer deletion", "Maintain a flat call hierarchy", "Question the threading", "Sweat the small leaks", and "The test".
• how: the explorer prompt drops its opening paragraph, the Exploration Instructions section, and the line above the output structure.
• poteto-mode: the babysit playbook drops the step 6 merge-authorization paragraph (step 9 still says babysitting never authorizes merging) and the closing Reply line. The worktree cleanup playbook drops its opening paragraph.
• why: four source playbooks get shorter. code-archaeology.md drops the PR-context step (the skill's seed-context step still pulls PR bodies), the evidence checklist, the pitfalls, and the return format. incident-postmortem.md keeps only the Linear, Slack, and Git bullets. sentry.md keeps "How to search it" and "What to return". slack.md keeps "Common pitfalls" and search strategies 1, 3, and 4, with their original numbers.
• Bumps pstack to 0.15.19.

No cross-references needed changes. Nothing in pstack links to a removed heading. The architect skill's citations of these principles still resolve, and architect still states "design it twice" and the three-file rule itself.

Test plan

• node scripts/validate-plugins.mjs passes.
• rg over pstack/ finds no link to or mention of a removed section.
• The edited files have no empty headings or double blank lines.