Add weekly TestFlight public-beta promotion

[jkmassel]

Summary

Adds the two beta-promotion tiers on top of the per-commit internal TestFlight builds (#25674), turning "release" into picking a build. Every promotion is a metadata-only App Store Connect call (distribute_only) — no rebuild.

per-commit  →  internal TestFlight
daily 00:00 →  last build of the day  →  Nightly Beta Testers   (automatic)
weekly      →  a developer picks a nightly  →  public beta        (human-chosen)

• Daily (automatic) — promote_nightly_build distributes the last processed build of the day to the Nightly Beta Testers group (WordPress + Jetpack).
• Weekly (human-picked) — gather_testflight_candidates lists the nightly builds in Slack and opens a Buildkite block step; promote_build ships the chosen build to public beta.

How it works

• Candidates come from App Store Connect, not reconstructed build codes — each carries its real marketing version, and the weekly list reads the Nightly Beta Testers group's members.
• Scoping — highest marketing version present (public beta tracks the latest line) → last 7 days → newest-first, enriched with the commit/PR from the matching Buildkite build.
• The pick — Slack presents the list and links to the block step's unblock dialog; the selection happens in Buildkite (block-step fields can't be submitted from Slack). WordPress and Jetpack share a build code, so one pick promotes both.

Safety

Both lanes distribute for real only on trunk (the scheduled branch); any other branch or local run defaults to a dry run that touches nothing (override with PROMOTE_DRY_RUN).

Not in this PR

• Buildkite schedules (managed in Terraform) — daily and weekly, both on trunk. Setup is documented in docs/testflight-promotion.md.
• Richer "what's new" from the PRs since the last public build — currently a minimal placeholder.
• Reader stays excluded (archive broken since Move PostHelper to app target #25321).

Testing

Validated end-to-end on CI via temporary branch-gated steps (since removed):

• Real nightly promotion → Nightly Beta Testers for WordPress + Jetpack (#32892)
• Weekly gather reads the nightly group + opens the block-step picker; empty group → "nothing to promote" (#32896, #32888)
• Latest-line + 7-day scoping and the dry-run gate — standalone logic tests
• Create the daily + weekly Buildkite schedules
• First real public promotion from trunk

[dangermattic]

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	32906
	Version	PR #25712
	Bundle ID	org.wordpress.alpha
	Commit	5d7328e
	Installation URL	50p7a8c8areuo

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	32906
	Version	PR #25712
	Bundle ID	com.jetpack.alpha
	Commit	5d7328e
	Installation URL	38hs53187cag8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.