ci(project): run CI on pull requests with any base

[ross-rl]

What

Drop the branches: [main] filter from the pull_request trigger in ci.yml, so check (Node 22 and 24: lint/format, typecheck, build, test) and audit run on every pull request regardless of its base. push stays scoped to main.

smoke-tests.yml keeps its branches: [main] trigger: its only job is already gated on startsWith(github.head_ref, 'release-please--branches--main') and it drives real devboxes, so broadening it would add cost without adding signal.

Why

Stacked PRs currently get no build, typecheck or test result at all — only conventional-commit. #155, #156 and #157 each landed with a one-item check list, which reads to a human reviewer as "verified" when nothing was actually run. Local verification is not a reproducible required check.

Cost is bounded: the concurrency group already cancels superseded runs per ref, and check/audit are a few minutes on a bun install.

Checklist

• PR title follows <type>(<scope>): <description> format (see above)
• bun run check passes (lint + format)
• bun run build passes
• bun run test passes
• SDK documentation updated (if applicable) — workflow-only change

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