Self-driving: configurable branch naming for agent PRs

Filing via the API, so I can't apply the enhancement label myself — please tag per CONTRIBUTING.

Is your feature request related to a problem?

Please describe.

Self-driving branches look like posthog-self-driving/fix<scope>-<description>-<hash> (older ones posthog-code/<description>). Our repo convention is <owner>/<ticket-id>-<short-description>, and some tooling keys off it.

We can't fix it after the fact: renaming the branch closes the PR. So we had to write an explicit exemption into our agent guidance saying the branch-naming rule doesn't apply to posthog[bot] — a carve-out in a hard rule, which is exactly the kind of thing that erodes a convention over time.

Describe the solution you'd like

A branch-name template setting with a few variables — something like {prefix}, {ticket}, {slug}, {hash} — so the branch can match the repo's convention.

This is much more useful alongside issue creation (PostHog/posthog#79133), since the ticket ID is usually the piece a naming convention wants. {ticket} is only substitutable if PostHog has minted the ticket.

Describe alternatives you've considered

• Renaming the branch after the PR opens. Not possible — it closes the PR.
• Relaxing our convention for everyone. Rejected; the convention exists so branches map to tickets and owners.
• The exemption we actually wrote. Works, but it's a documented hole in a rule we otherwise enforce.

Additional context

Low priority relative to PostHog/posthog#79131 and #79132 — this one is cosmetic, those two cost real time.

Debug info

• PostHog Cloud (US)

Start by locating the self-driving branch-name construction entry point and read the related issue, PostHog/posthog#79133, for ticket creation context. Trace how current prefixes, descriptions, and hashes are assembled. Done means a documented template setting supports the requested variables and handles ticket substitution only when PostHog has minted a ticket.