fix(merge-orchestration): never choose rebase; a stale :rebase decision squashes

[hyperpolymath]

What

Hypatia's merge orchestration no longer chooses or emits a rebase merge.

• Strategist.decide_method: removed the commits_atomic_green -> :rebase clause. An atomic-green series is squashed; a :proof PR still gets :merge_commit.
• BatonEmitter: removed gh_flag(:rebase). A manifest written before this change now falls through to --squash rather than emitting a flag the repo refuses.

Why

Rebase-merge replays commits unsigned. Of 286 unsigned commits on estate default branches, 119 were rebase replays, proven by patch-id twins against their signed feature-branch originals. Those replays break required_signatures.

Owner ruling (2026-09-30): allow merge commits but not rebase. Rebase is being disabled repo-side (allow_rebase_merge=false, already applied to hypatia, standards and launch-scaffolder). This PR stops the orchestrator from asking for a method the repos now refuse.

Verification

• mix test test/merge_orchestration: 82 tests, 0 failures.
• MIX_ENV=test mix compile --warnings-as-errors: rc=0. mix format --check-formatted on the changed files: rc=0.
• Mutants: restoring the strategist's rebase clause turns the new strategist test red, and restoring gh_flag(:rebase) turns the emitter test red. The first draft of the strategist test used the helper's default :chore class; it was squashed by the first clause and passed vacuously. The surviving mutant caught this.

Inherited red (not required, not introduced here)

• governance / Actions lockfile verify: red on main at 300fd45 as well, from dependabot bumps that left actions.lock stale. Tracked in actions.lock drift on main: dependabot bumps left 4 refs + codeql SHA unlocked (Actions lockfile verify red) #885.

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📥 Commits

Reviewing files that changed from the base of the PR and between 300fd45 and f1e9757.

📒 Files selected for processing (4)

• lib/merge_orchestration/baton_emitter.ex
• lib/merge_orchestration/strategist.ex
• test/merge_orchestration/baton_emitter_test.exs
• test/merge_orchestration/strategist_test.exs

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