[Infra] Pin squash-only merges with PR-title messages in bootstrap

[DongyuZhao]

Summary

• The template's embedded bootstrap-repository.sh now pins the merge policy: squash-only (allow_merge_commit=false, allow_rebase_merge=false) with squash_merge_commit_title=PR_TITLE + squash_merge_commit_message=PR_BODY, so both direct merges and merge-queue commits land as title (#N) + PR description. The PATCH sits between the default-branch validation and the ruleset upserts, matching the tex-core placement, and the re-run stays idempotent.
• Ported from tex-core ([Infra] Pin squash-only merges with PR-title messages in bootstrap tex-core#20), the same direction as [Release] Refresh Maven checksum sidecars after the publication merge #36/[Infra] Preserve benchmark failures through the metrics tee #37. tex-core changed both its checked-in scripts/bootstrap-repository.sh and its template's embedded copy; markdown-core carries the script only as the embedded copy in docs/repository-setup-template.md, so this PR touches that one spot.
• No CHANGELOG entry: markdown-core's changelog records release changes only (no Unreleased section), matching [Release] Refresh Maven checksum sidecars after the publication merge #36/[Infra] Preserve benchmark failures through the metrics tee #37.

Verification

shellcheck clean on the extracted embedded script; audit:repository and audit:ci green.

🤖 Generated with Claude Code

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PR performance and binary-size diff

Benchmark execution is required; hosted-runner timing, memory, and size deltas are informational only.

Baseline: exact PR base f126d5ffb340 (successful main CI).

Performance

Runtime	Workload	Median	Perf diff	Memory	Memory diff
c	representative_large	6.012 ms	⚠️ +31.2%	8,560 KiB	✅ -2.1%
es	deep_nesting	0.287 ms	✅ -24.0%	156,896 KiB	⚠️ +17.4%
es	large_document	38.112 ms	✅ -5.0%	156,896 KiB	⚠️ +17.4%
kotlin	deep_nesting	0.213 ms	⚠️ +18.3%	95,580 KiB	✅ -5.6%
kotlin	large_document	28.075 ms	⚠️ +32.9%	94,008 KiB	✅ +0.8%
swift	deep_nesting	0.079 ms	✅ -47.0%	26,144 KiB	✅ +0.2%
swift	large_document	13.554 ms	✅ -3.9%	26,080 KiB	✅ +0.1%

Binary size

Artifact	Bytes	KiB	Size diff
c-shared-library	501,888	490.1	✅ 0 B (0.0%)
es-wasm	288,573	281.8	✅ 0 B (0.0%)
kotlin-jvm-jar	312,972	305.6	✅ 0 B (0.0%)

Head CI run

Size deltas are deterministic for identical build inputs. Performance deltas compare separate hosted runners and should be treated as directional evidence, not a pass/fail threshold.

[DongyuZhao]

Extended in 00abd2f: the embedded recipe now carries the live merge queue (SQUASH/ALLGREEN) and the repository-admin bypass behind MERGE_QUEUE/MAIN_ADMIN_BYPASS variables (a re-run previously would have stripped both), and the merge-policy wording now describes the squash title/message settings as repository defaults (applied mechanically by queue commits, editable on direct merges) per the review on the tex-core twin PR. The live repository settings were switched to squash-only with PR-title/description defaults via the same call.