fix(autocoder): resolve merge conflicts before escalating, not on first failure

[laird]

Problem

merge-to-integration.sh aborted the merge and escalated (removed working,
added needs-clarification, posted a hard-escalation comment) on the first
merge conflict, before the /fix worker that had just written the fix — and
was sharing the exact same checkout — ever got a chance to look at it.

Reported as ey-org/athena2#1766, with two concrete instances 15 minutes apart
in one session where the worker read the failure and resolved the conflict by
hand anyway (mechanical conflicts it had full context for), after the label
had already flipped to needs-clarification.

Why it matters: needs-clarification is in the blocking set, so between the
premature escalation and the worker's own recovery the issue is invisible to
the claimable queue. If the worker's loop ends before it finishes (context
exhaustion, restart, gate failure), the issue is stranded wearing a
human-blocked label while a complete fix sits unmerged on its branch — and the
label is not self-clearing.

Fix

• merge-to-integration.sh: on conflict, leave it ON DISK (no
  git merge --abort), don't touch labels, post an advisory-only comment, and
  exit 3 — a code distinct from 1 (push failure) and 2 (test failure).
• merge-poll.sh: documents the new exit code 3 (already passed through
  generically, no behavior change needed there).
• commands/fix.md: at all 3 merge sites, exit 3 now triggers a direct
  self-resolve attempt (the worker resolves the conflict using the context it
  has, completes the merge commit, and re-launches). Only if that second
  attempt also fails does it escalate to needs-clarification — via the
  same explicit-release-comment-then-label convention already used elsewhere
  in the file (fixes a tests/test_fix_working_lock.py invariant that a
  working-lock release must have an adjacent terminal-outcome marker).
• Bumps the autocoder plugin to 4.21.0 across every manifest copy
  (tests/test_manifest_versions.sh enforces they all agree).
• Adds tests/test_merge_conflict_advisory.sh: a real git-fixture test that
  drives an actual merge conflict through merge-to-integration.sh and
  asserts exit 3, the conflict left unresolved on disk, no label-changing
  issue_update call, and an advisory (not hard-escalation) comment body.

Test plan

• tests/run-shell-suite.sh — 27/27 passed (includes the new test and the
  existing test_merge_launch_poll.sh, which still passes with the extra
  launch/poll call sites at each merge site)
• pytest tests/ — 116/116 passed (including
  test_fix_working_lock.py, which specifically checks every
  --remove-label "working" site has an adjacent terminal marker, and
  test_gate_md_bash.py, which checks fix.md's bash blocks parse)
• tests/test_manifest_versions.sh — all manifests agree on 4.21.0