skills: t27o -- the night lessons (review checklist, drafts hold PRs, sweep and the Zig front end, build traps) #10826

Goal

The t27o skill carries what the overnight loop learned after #10542, so the next loop (and any agent changing t27o) does not pay for the same traps again. Part of #10522; the owner asked for experience to be kept in the skills.

Deliverable

.claude/skills/t27o/SKILL.md: the Zig front end and tri t27o-sweep (--repo) in "Where the truth lives" and "Measure coverage"; the review checklist (dupe_scan for bodies, the own-language gate compiled from the merge base, devhome, gen byte-identity, seals of specs that use rust_convert); drafts as the only reliable hold; the stacked-PR rebase recipe; traps: the cloned writable CARGO_HOME, a private ZIG_LOCAL_CACHE_DIR per lane, zsh echo / = / backticks, the gen-rust splice matching string literals (#10824), worktree disk cost; stale numbers updated (inner loop, compiler.rs time).

Done when

python3 -I tools/check_documented_commands_exist.py is ok (138 dead tri mentions = the ceiling, with a tri that has t27o-sweep); ASCII only.

Boundary

• May touch: .claude/skills/t27o/SKILL.md only.

Links

#10522, #10541, #10801, #10646, #10819, #10824.

User Scenarios

• Given the overnight loop has encountered traps in the Zig front end, tri t27o-sweep, review checklist items, draft PR holds, and stacked-PR rebasing, when an agent reads .claude/skills/t27o/SKILL.md, then they find documented lessons for each trap so the next loop does not repeat them.
• Given the skill document must pass the documented-commands check, when running python3 -I tools/check_documented_commands_exist.py, then it exits 0 with 138 dead tri mentions as the ceiling and tri having t27o-sweep.

Requirements

• FR-001: The skill document MUST be created at .claude/skills/t27o/SKILL.md as the only file touched.
• FR-002: The document MUST contain a "Where the truth lives" section covering the Zig front end and tri t27o-sweep (with --repo).
• FR-003: The document MUST contain a "Measure coverage" section.
• FR-004: The document MUST contain a review checklist including: dupe_scan for bodies, the own-language gate compiled from the merge base, devhome, gen byte-identity, and seals of specs that use rust_convert.
• FR-005: The document MUST document drafts as the only reliable hold for PRs.
• FR-006: The document MUST include the stacked-PR rebase recipe.
• FR-007: The document MUST list traps: cloned writable CARGO_HOME, private ZIG_LOCAL_CACHE_DIR per lane, zsh echo / = / backticks, gen-rust splice matching string literals ([bug] t27c gen-rust: the use splice matches words inside string literals and pulls in a whole module (13,432 vs 4,111 lines) #10824), and worktree disk cost.
• FR-008: The document MUST update stale numbers for inner loop and compiler.rs time.
• FR-009: The document MUST be ASCII only.
• FR-010: After the document is written, python3 -I tools/check_documented_commands_exist.py MUST exit 0.