add /ctx-task-out — close the spec→implement gap in the canonical chain

[josealekhine]

add /ctx-task-out: close the spec→implement gap (+ Claude 5 context-window fix)

Summary

The design-to-implementation chain had an unowned artifact: /ctx-plan
disclaims implementation planning, /ctx-spec commits the what/why at
spec altitude, and /ctx-implement opens with "use when you have a
plan document" — which nothing in the chain produced. This PR adds the
missing fourth step:

/ctx-brainstorm → /ctx-plan → /ctx-spec → /ctx-task-out → /ctx-implement
    (vague)     (contested)  (committed)   (decomposed)     (execution)

/ctx-task-out decomposes a committed spec into a per-milestone plan
at specs/plans/<milestone>.md — data model, contracts, invariant test
matrix, and commit-sized tasks with falsifiable acceptance criteria.
Its signature move matches the rest of the chain: it refuses to run
ahead of its inputs (blocking-TBD gate, rolling-wave gate, milestone
boundaries owned by the spec) rather than degrading. The plan is the
execution ledger; TASKS.md carries epic-level anchors only.

Also included: a fix for Claude 5 family context-window detection —
claude-fable-5 sessions fell to the 200k default and the
check-context-size hook warned "104% full" on a 79%-free 1M session.

Commits

1. 7c1eb0e3 — add /ctx-task-out (Spec: specs/ctx-task-out.md)
   • New skill, claude + copilot-cli parity (generated via
     hack/sync-copilot-skills.sh), registered in the permissions
     allowlist and ctx init workflow tips
   • 5-step chain across skills, playbook templates, and docs
   • /ctx-spec: tasking handoff in interactive and --brief flows
   • /ctx-implement: names specs/plans/<milestone>.md as canonical
     input, redirects bare multi-milestone specs, refuses
     Status: Blocked plans, owns plan checkbox updates
   • Project template specs-README lifecycle gains the task-out step
   • Docs: skills reference (including the previously missing
     /ctx-plan section and its dead anchor), common-workflows,
     integrations, three recipes
2. 500c4a07 — fix context window detection
   (Spec: specs/model-context-window-fable.md)
   • ModelContextWindow maps the always-1M families: fable,
     mythos, sonnet-5 (alongside existing opus and [1m])
   • sonnet-4-6 deliberately unchanged (Claude Code gates its 1M
     behind the [1m] opt-in — see spec Non-Goals and DECISIONS.md)
3. f37fd9fe — chore: refresh go.work.sum
   (Spec: specs/meta/chores.md)

Design notes for reviewers

The skill went through two external review rounds before landing.
Load-bearing pieces to scrutinize, in the order the work order's
Review section prescribes:

1. Gate behavior — blocking-TBD refusal, rolling-wave refusal,
   Status: Blocked reachable only via amendment (fresh runs refuse
   and write nothing)
2. Authority discipline — decompose, don't redesign the bet;
   invariants come from the spec (unstated invariant = spec gap, mark
   TBD, never mint); TBD resolutions live in the spec or DECISIONS.md,
   the plan only points at them
3. Execution ledger — plan owns task/DoD checkboxes,
   /ctx-implement updates them, rolling-wave gate reads DoD only
4. Amendment mode — obsolete-and-append, ids never reused,
   acceptance criteria never edited in place once a task starts
5. Chain-text consistency — grep for 4-step remnants returns none

Verification

• Full gate green pre- and post-commit: build, go vet, gofmt,
  lint-drift, style/version/why/copilot-sync checks, go test ./...
  zero failures (golangci-lint runs in CI; not installed locally)
• gitnexus detect_changes: 66 symbols across 21 files, 0 affected
  execution flows, risk LOW
• Window fix verified by table test (fable/mythos/sonnet-5 → 1M;
  sonnet-4-6 → 200k pinned)
• Blocking-TBD gate verified read-only against zhc/os
  specs/v1-substrate.md (D-001 open → refusal fires by construction)

Not in this PR / follow-ups

• site/ regeneration (make site) — generated output, rebuilt
  separately
• Reinstall (make build && sudo make install) so the live hook
  stops misreporting and the plugin picks up the skill
• First real /ctx-task-out run: zhc/os v1-substrate.md m0a
  (expected: D-001 refusal; then the plan after D-001 resolves)
• docs/reference/skills.md drift sweep: 7 skills still
  undocumented (ctx-architecture-enrich, ctx-code-review,
  ctx-config, ctx-dream, ctx-explain, ctx-refactor,
  ctx-serendipity) plus a stale ctx-prompt row
• allow.txt / write.yaml omit /ctx-plan itself (pre-existing)