feat(skills): support agent-neutral interactive skills

[rochecompaan]

Summary

• restore and expand the agent-neutral interactive-skills design omitted from Split interactive planning into user-global Patchmill skills #132, with a detailed implementation plan
• describe installation and invocation generically while retaining clearly labeled Pi path and command examples
• generalize all four packaged activation contracts from Pi-only to compatible human-controlled interactive coding-agent sessions
• preserve print-only, RPC, unattended, and automated stop boundaries plus every existing planning, publication, label, and cleanup safety rule

Compatibility audit

The complete patchmill-plan, patchmill-upload, patchmill-label, and patchmill-cleanup bodies contain no Pi runtime API, environment variable, filesystem path, or command dependency. Their operations use Agent Skills Markdown, conversation context, repository files, Git, patchmill, and the configured gh or tea provider CLI. Pi remains one concrete compatible agent rather than a requirement.

Context

PR #132 was squash-merged before original design commit 0c57e60acc24 was pushed. This PR first recovered that artifact, then implemented the approved coding-agent-neutral goal after final review identified the four old Pi-only activation guards.

Verification

• writing-skills RED/GREEN pressure cycles for each skill: five current-contract Pi-only refusals, five compatible-agent interactive successes, and five unattended refusals per skill
• all four revised skills load together without frontmatter, format, or name-collision errors
• npm dry-run includes all four; recommended/project skill packs exclude them; configured planning skill and siblings resolve
• npm test — 1,154 passed, 0 failed
• npm run lint — 0 errors
• npm run build
• npm run site:build — 18 pages, including /using-patchmill/interactive-skills/
• git diff --check origin/main...HEAD

No automated test was added merely to assert static Markdown text; behavioral pressure evidence was saved outside git and direct integration checks cover packaging and loading.