Harden Dependabot metadata and branding footer automation

[ashleyshaw]

Automation Summary

Harden the repository automation so Dependabot updates stop inheriting per-PR milestones, while the branding agent reliably applies a footer across Markdown documents and the supporting docs/workflows stay in sync.

Steps / Checklist

• Stop auto-creating unique milestones for each Dependabot PR.
• Keep Dependabot dependency PRs on a shared auto-merge path with squash merge and branch update support when checks require it.
• Restore consistent footer insertion for Markdown docs through the branding/meta agent path.
• Update workflow/config/instruction docs so the behaviour is durable and discoverable.
• Validate the change with repo tests and workflow/config checks.

Acceptance Criteria

• Dependabot PRs no longer receive a unique milestone per PR.
• Dependabot PRs continue to auto-update and auto-merge where checks pass and conflicts do not block them.
• Markdown docs are branded consistently through the repo automation path.
• Footer/branding docs accurately describe the live implementation.
• Tests and validation commands pass.

Additional Context

The current automation appears to create milestones from PR titles when none exist, which causes milestone sprawl for Dependabot updates. The branding agent is present, but the footer helper and workflow wiring need to be audited so docs such as docs/BRANCHING_STRATEGY.md receive the expected branded footer.

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Definition of Ready (DoR)

• Automation goal and scope defined
• Checklist prepared
• Estimate added

Definition of Done (DoD)

• All checklist and acceptance criteria completed
• Documentation/changelog updated
• Approved by maintainer

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[linear-code]

LS-1105

[github-actions]

✅ Template check passed after update. Thanks for fixing the issue body.