ci: run CI on pull requests to any base branch

[cunninghamcard-bit]

Requested by li · project thread

Before: CI ran only on pull requests whose base is master. A stacked pull request, whose base is the branch of the pull request below it (for example #250 on #249), got no checks at all until everything under it had merged.

After: every pull request runs CI, whatever its base. Since #263, changes diffs the test merge commit against its first parent, the base branch head, so a stacked pull request runs only the jobs its own layer needs.

Running on every layer means one push low in a stack queues a run on each pull request above it, so a new push to a pull request now cancels that pull request's older run. Pushes to master each get their own concurrency group: GitHub also cancels a pending run in a group when another one queues, and changes diffs a push only against the one before it, so a documentation push queued behind a cancelled code push would have left that code untested on master.

This was split out of #262 so it can be reviewed on its own. #262 and #249 carry the same lines, byte for byte, so after this merges they merge with master without a conflict, and their diffs no longer show it.

How

• on.pull_request loses branches: [master].
• A top-level concurrency block: group ci-<workflow>-<PR number or commit SHA>, cancel-in-progress only for pull requests.

Checks

• actionlint is clean.
• Merging this into master and then ci: speed up test builds, deduplicate caches, and use prebuilt cargo-ndk #262's current head gives the same tree as merging ci: speed up test builds, deduplicate caches, and use prebuilt cargo-ndk #262's head alone.
• The trigger change and the per-pull-request cancellation have run on ci: speed up test builds, deduplicate caches, and use prebuilt cargo-ndk #262 and on the stacked [1/8] chore: CI on every PR and test fixture cleanup #249–[8/8] refactor!: public JSON equals the AI SDK JSON #256 since 2026-10-14. The per-commit group for master pushes is new here; before, master pushes shared one group per branch.