feat(sweep): add loom:operator-only label + pre-flight skip

[rjwalters]

Summary

• Adds loom:operator-only label to .github/labels.yml (orange-500, groups with loom:abort).
• Sweep pre-flight (Wave Lifecycle step 1) now skips operator-only issues with a logged reason; Constraints prose now references the mechanism instead of an unenforced promise.
• Champion --merge-mode guard added so loom:curated + loom:operator-only issues are never silently auto-promoted into the build queue (the critical safety guard — without it the sweep skip is bypassed under --merge).
• loom.md labels table documents the new label between loom:blocked and loom:urgent.
• Defensive operator-only check added to /shepherd Step 3 so direct shepherd invocations exit cleanly instead of stalling.
• Curator intentionally unchanged — /curator N on an operator-only issue should still enrich the body; the constraint is "no builder claims it", not "no curation happens".

Note: .claude/commands/loom/*.md is hardlinked to defaults/.claude/commands/loom/*.md (same inodes), so a single edit updates both trees. Verified with stat -f "%i %N".

Closes #3360

Test plan

• /sweep N where #N has loom:operator-only → builder NOT dispatched; log shows skip with reason "operator-only".
• /sweep N where #N lacks the label → unchanged behavior.
• /curator N on a loom:operator-only issue still enriches the body (does not skip).
• Champion in --merge mode does NOT auto-promote loom:curated + loom:operator-only issues.
• /shepherd N on a loom:operator-only issue exits with the new message instead of writing a spawn signal.
• After ./scripts/install/sync-labels.sh ., gh label list | grep loom:operator-only returns the new label (deferred to operator; not run as part of this PR).

[rjwalters]

LGTM. Approved.

Verified:

• .github/labels.yml: New loom:operator-only entry in Status Labels group adjacent to loom:abort (same color F97316), kebab-case naming, description includes Applied by: clause. YAML parses to 26 valid label entries.
• sweep.md Wave Lifecycle step 1: pre-flight skip correctly placed after loom:blocked with explicit reason logging "operator-only". The pre-flight jq query was updated from state → state,labels to support the new check.
• sweep.md Constraints: the previously unenforced "Skip operator-only issues" promise now references "see Wave Lifecycle step 1" — promise → mechanism.
• sweep.md Limitations table: loom:operator-only enforcement marked Implemented (/sweep: add loom:operator-only label so pre-flight can actually skip operator-only issues (constraint exists but is unenforceable) #3360) with accurate scope note covering the Champion --merge guard too.
• loom.md labels table: loom:operator-only row placed between loom:blocked and loom:urgent per curator's spec, Set by = Human.
• champion-issue-promo.md (critical guard): loom:operator-only added to "When NOT to Auto-Promote (Even in Force Mode)" — without this, --merge mode would silently auto-promote loom:curated + loom:operator-only issues, completely bypassing the sweep pre-flight skip. Reasoning prose explains why promotion would stall the queue.
• shepherd.md Step 3: defensive skip added with clean exit message, "regardless of --merge" caveat is correct, jq query updated to fetch labels alongside state.
• curator.md: unchanged, as the curator notes explicitly directed (/curator N on operator-only issues should still enrich the body).

Scope discipline:

• Exactly 5 files modified (+27/-4) — matches PR description with no scope creep.
• No code files touched (no Rust/TS/Python edits); pre-existing local pnpm check:ci clippy failures in loom-daemon/src/init/ are not caused by this PR and the GitHub Rust jobs are appropriately skipped via path filter.
• No gh label sync invoked from any script — operator gets to choose when to sync to the live repo. PR body explicitly defers that step. Good.
• Hardlink claim in PR body verified: defaults/.claude/commands/loom/sweep.md and .claude/commands/loom/sweep.md share inode 684242883, so the single edit propagates to both trees.
• CI status: CLEAN merge state, all required GitHub checks pass (Installer Integration Tests, Detect Changes), Rust jobs correctly skipped.

The critical design insight is preserved: the constraint is "no builder claims it", not "no curation happens". Curator stays enabled, sweep/shepherd/champion-force-mode all skip. The defense-in-depth across three skip points (sweep pre-flight, shepherd direct invocation, champion force-mode promotion) is exactly right.

[rjwalters]

Rebased onto main (#3361 merged). Resolved conflicts in sweep.md pre-flight + Limitations table. Both skip checks now compose — operator-only first, then existing-PR probe.

[rjwalters]

Re-approved after rebase. Both skip checks compose correctly.

Verified post-rebase:

• Pre-flight skip section in sweep.md (step 1): closed → building → blocked → loom:operator-only → existing-PR probe. Order is right, with explicit "Checked before the existing-PR probe" note so operator-only issues aren't probed.
• Limitations table: BOTH rows present — existing-PR detection (/sweep: detect existing open PRs in pre-flight; route to Judge instead of spawning a duplicate Builder #3359, Implemented) AND loom:operator-only enforcement (/sweep: add loom:operator-only label so pre-flight can actually skip operator-only issues (constraint exists but is unenforceable) #3360, Implemented).
• Pre-flight skip rule callout updated to list loom:operator-only among skip reasons.
• No regressions in .github/labels.yml, champion-issue-promo.md, loom.md, shepherd.md. Bottom-of-file "Skip operator-only issues" bullet correctly references the label and points back to Wave Lifecycle step 1.
• No accidental new behavior beyond the conflict resolution.
• CI passing, mergeStateStatus CLEAN.