v1.91.287.0 feat(write-skill): write, change and diet skills; /learn at half its template size (#3524)

* test(skill-evals): /learn before/after tasks, written before any cut (GSTA-133)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(learnings): --help, unknown-flag errors and --stats on the learnings bins; cross-project search no longer drops every project when the state root path contains the slug

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* refactor(learn): diet /learn onto the levels: tool flags replace the inline stats program, one mode table, prune rules kept

Skill-Diet: learn
Skill-Size: 5797→2586 bytes (SKILL.md.tmpl)
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* feat(write-skill): /write-skill skill, gstack-skill-bill, static skill-to-bin drift ratchet, shared skill eval contract

Skill-Size: 0→3245 bytes (write-skill/SKILL.md.tmpl)
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(learn): guard the state-root lookup in Prune; re-measure the learn ceiling

Skill-Size: 2586→2707 bytes (learn/SKILL.md.tmpl)
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix: pre-landing review fixes (drift scanner, learnings flags, skill-bill baselines, eval fixtures)

- skill-bin-drift: a redirect no longer ends a command, so `bin/x 2>/dev/null --bogus` is checked
- gstack-learnings-search: a missing value or a non-positive --limit exits 2 like an unknown flag
- gstack-skill-bill: the gstack root skill is a checkout skill; Skill-Diet trailers match the exact name; the baseline counts the committed bytes untrimmed
- skill-evals task files get a free contract test and are registered in FREE_FIXTURES, with the drift allowlist
- learn eval: the no-code-change safety task no longer asks the model to fix code (its check penalised obeying the user)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(learn,write-skill): full bin paths in the mode table; prune asks per entry unless the request names the action

Skill-Diet: learn
Skill-Size: 2707→2955 bytes (learn/SKILL.md.tmpl)
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* docs(changelog): /write-skill, gstack-skill-bill, drift ratchet, eval contract, /learn diet

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* docs: drift ratchet in CONTRIBUTING's static tests; /learn prune example asks per entry

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(skill-bill): report a checkout skill's eval task file and check it against the contract

The skill-evals task-file test now exercises lib/skill-bill.ts's checkEvalTasks, with a negative control, instead of a test-only validator (test-of-test ratchet).

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(skill-bill): EVAL TASKS path uses forward slashes on every platform; a non-object task entry is a problem, not a crash

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

* fix(learn): BASH_COMPAT guard on in-window heredocs; count skill-e2e-write-skill in the paid census

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Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

gstack/llms.txt

@@ -71,6 +71,7 @@ Conventions:
 - [/test-audit](test-audit/SKILL.md): Find low-value or duplicate tests and the test-only code they keep alive.
 - [/ubicloud](ubicloud/SKILL.md): Run a heavy suite, build or eval on an ephemeral 16-vCPU Ubicloud VM: TTL-named VMs, a reaper, quota preflight and a hang watchdog.
 - [/unfreeze](unfreeze/SKILL.md): Clear the freeze boundary set by /freeze, allowing edits to all directories again.
+- [/write-skill](write-skill/SKILL.md): Write a new agent skill or improve one without bloat: the edit ladder, levels, drift and size checks, full diets.
 
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