Scan v2/main with CodeQL: switch from default setup to a committed advanced-setup workflow

[cliffhall]

Problem

CodeQL runs as default setup (actions, javascript-typescript, weekly), which analyzes only the default branch (main) and PRs into it. Every analysis on record is refs/heads/main or one of the four milestone-merge PRs (#2304, #2381, #2456, #2536). No v2/main work is scanned until the release PR.

That is how the v2.9.0 ReDoS (#2540) surfaced: CodeQL flagged a regression from this milestone's #2423 work only on the milestone-merge PR, and the release was held while it was fixed. The finding should have landed on the PR that introduced it.

Change

Switch to advanced setup: a committed .github/workflows/codeql.yml with

• push to main and v2/main
• pull_request against main and v2/main
• the existing weekly schedule
• the same languages (actions, javascript-typescript) and the default query suite

Disable default setup in the same change, or the two will double-report.

The workflow's analyze job holds security-events: write, which makes it a credentialed job, so every action it uses must be SHA-pinned (verify:action-pins, #2484) and it gets a timeout-minutes sized from observed runs (AGENTS.md).

This needs no new credential and does not depend on the GitHub App.

Acceptance

• Code scanning shows analyses for refs/heads/v2/main, and for a PR targeting v2/main
• Default setup is disabled, with no duplicate alerts
• verify:action-pins passes, and the job declares timeout-minutes

Part of tracker #2543. Needs no credential; can start now.

[cliffhall]

Triage: Priority Medium (total 8)

• Severity 3 — v2 code is not scanned until the release PR
• Urgency 3 — wanted this milestone; needs no credential
• Bonuses: +1 milestoned (v2.10.0), +1 sub-issue of an epic (Tracker: security and board automation on a GitHub App credential #2543)

Board: #28, Status Todo. Part of tracker #2543.