ci: run CI on rc branch PRs and pushes

[chr1syy]

Problem

.github/workflows/ci.yml only triggers on main:

on:
  pull_request:
    branches: [main]
  push:
    branches: [main]

So PRs targeting rc get no real CI run at all. CodeRabbit is the
only automation that fires, and it skips non-default branches — which
means an rc PR can look "green" while nothing has actually run tsc
or the test suite.

This bit us on #67 (inbound thread-session binding, including the
owner_user_id gate that fixed the session-hijack hole): it merged into
rc with zero CI checks, and the type-check + 579-test run had to be
reproduced by hand locally before merging.

Change

Add rc to both branch filters. One-line-per-trigger change, no job or
step edits — rc PRs and pushes now get the exact same npx tsc --noEmit + npm test gate as main.

Notes

• No effect on main: its triggers are unchanged.
• No new jobs, no new secrets, no runner-cost surprise beyond the rc
  PRs that should have been running all along.

🤖 Generated with Claude Code

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