feat(wiki): document session-handoff skill design rationale

Capture the design decisions behind the session-handoff skill in a dedicated
`.wiki/concepts/session-handoff-skill-design.md` page, indexed and logged per
Karpathy LLM Wiki conventions. Covers the problem statement (cold-start fog
between CC sessions), the sliding-overwrite contract for `.tasks/NEXT_SESSION.md`,
the phrase whitelist + anti-pattern guards + ambiguity-asks-not-guesses rule,
the substantive-commit heuristic (prefix exclude AND (body>200 OR files>3) +
first-non-trivial-commit-always exception), the opt-in PostToolUse hook that
replaces agent-side memory with harness-side determinism, the orient+ask read
mode default, project scope guarantee, the five-section handoff content contract,
secret-detect abort, staleness-7d query, mid-task capture, precedent comparison
against using-tasks / MEMORY.md / log.md / Karpathy diary, and the closure
record of cluster 7/7 (install / hermes-mapping / bootstrap-template-extend /
posttooluse-hook / existing-projects-upgrade / test-trigger / review). Source
buffer: `~/projects/.workshop/.archive/2026-05-24-session-handoff-skill.md`
Round 1 design + Round 2 Q1–Q10 resolution.

Also serves as the live-hook e2e smoke artifact for the deferred follow-up from
`[session-handoff-posttooluse-hook]` — substantive commit (feat: prefix, body
well above 200 chars) should trigger the PostToolUse `commit-detector.ps1` hook
just enabled in `~/.claude/settings.json` and emit `hookSpecificOutput.additionalContext`
on the next agent turn.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.wiki/index.md

@@ -39,6 +39,7 @@ Catalog of all wiki pages. One line per page, organized by type. Updated on ever
 - [tdd-criteria-design](concepts/tdd-criteria-design.md) — tdd-criteria-design
 - [project-bootstrap-meta-isolation.md](concepts/project-bootstrap-meta-isolation.md) — project-bootstrap@1.11.0 — Step 1 ships meta-isolation block in `.gitignore` (`!.claude/`, `!.tasks/`, `!.wiki/`, ...) so own greenfield/upgrade projects re-enable agent meta-paths against global `core.excludesFile` cutter. Marker-based append-only on existing files; smoke-tested with negative control
 - [interns-grep-audit-design](concepts/interns-grep-audit-design.md) — interns-grep-audit-design
+- [session-handoff-skill-design.md](concepts/session-handoff-skill-design.md) — design rationale for the `session-handoff` skill (sliding overwrite into `.tasks/NEXT_SESSION.md`, phrase whitelist + substantive-commit heuristic, optional PostToolUse hook for harness-side determinism, orient+ask default, project scope, cluster 7/7 closure)
 
 ## Packages