feat(code): add commit trailers for attribution

[adboio]

Problem

we don't (yet) do a great job of attributing work done by posthog code back to posthog code.

one small step in that direction is adding commit trailers to all commits made in our UI

Changes

adds commit trailers to all commits from the UI, with a reference to posthog code and the task ID

How did you test this?

tested manually, sample git log:

commit 2c95d4396cc3fed3f138b4f0e63e8f771632c007 (HEAD -> posthog-code/test-change)
Author: Adam Bowker <adam.b@posthog.com>
Date:   Wed Apr 1 14:19:10 2026 -0700

    fix(actions): remove test comment from ActionsModelProps interface

    Generated-By: PostHog Code
    Task-Id: e6f79d08-9b9d-42fe-8648-14fe1aad8bcb

[adboio]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(code): persist PR URLs to backend & emit file activity events #1434
• feat(code): add attribution to PR body from UI #1433
• feat(code): add commit and pr attribution to system prompt #1432
• feat(code): add commit trailers for attribution #1431 👈 (View in Graphite)
• Add staged file tracking and selective commit support #1429
• Fix git diff to compare against HEAD instead of working directory #1422
• main

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