chore: retire dev as a standing integration branch (ADR-0022)

[jack-champagne]

What

Supersedes ADR-0021's branch model with ADR-0022. Mechanical parts of
the reconciliation already done directly (not code, so no diff here):

• GitHub default branch: dev -> main
• Retargeted the 3 PRs that had silently targeted dev (ci: engine typecheck gate + fix dev's engine type errors (#1228) #1235, feat(session): Pause/Resume engine primitive — Paused durable marker + foreground sentinel (#1221) #1226, chore(deps-dev): bump the dev-dependencies group across 1 directory with 6 updates #577) to main

This PR's diff:

• ci.yml: drop dev from the push trigger
• docs/adr/0021-...md: amended with a superseded-status note
• docs/adr/0022-retire-dev-branch.md: the new record — full rationale,
  evidence, and what's kept vs retired

Why (short version — see ADR-0022 for the full case)

• dev regressed a shipped fix within 48 hours of ADR-0021 landing (wave-2's
  back-merge silently dropped the fix(app-bundle): drop messages with empty content array before API dispatch #1218 guard; CI stayed green because no CI
  job runs the overlay's own test suite). See fix(engine): port the #1218 empty-content guard onto main (currently only on dev) #1240 for the port back onto
  main.
• dev was also the GitHub default branch — not just a git ref — which is
  why PRs opened without an explicit --base (including the fix for the
  regression above) and Dependabot silently targeted it instead of main,
  contradicting CONTRIBUTING.md.
• No canary implementation was ever built against it — the only artifact
  from ADR-0021 was this same ci.yml trigger line.
• The batching value a wave-branch provides doesn't need a standing
  branch — wave-2's actual peak divergence was 18 commits over ~1.5 days,
  which an ephemeral integration/wave-N branch (created per batch, deleted
  on merge) gets identically.

What's NOT rejected

The canary concept itself — continuous testing of a real in-place upgrade on
persistent fleet state, which CI's disposable runners structurally can't do —
is retargeted at main's own CI-green commits/tags, not deleted. ADR-0022
has the build-out sketch.

Sequencing

1. This PR + the default-branch flip + PR retargets (done)
2. fix(engine): port the #1218 empty-content guard onto main (currently only on dev) #1240 — port the fix(app-bundle): drop messages with empty content array before API dispatch #1218 guard onto main
3. Delete origin/dev once fix(engine): port the #1218 empty-content guard onto main (currently only on dev) #1240 lands (its tip SHA will be recorded here
   first, so it's trivially recoverable if anyone needs it)

For Aaron/JJ

This reverses ADR-0021 two days after it landed — flagging explicitly
rather than landing it as a fait accompli. Happy to talk through the
evidence in the ADR if any of it reads differently from where you were
sitting when you wrote it.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 2f466d6 and 6910a14.

📒 Files selected for processing (3)

• .github/workflows/ci.yml
• docs/adr/0021-canary-fleet-arms-ci-artifacts.md
• docs/adr/0022-retire-dev-branch.md

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