Deduplicate PR CI runs (Gate only)

[stranske]

This removes redundant PR CI runs and cuts the PR matrix:

• .github/workflows/ci.yml: no longer runs on pull_request (push-only)
• .github/workflows/pr-00-gate.yml: runs python-versions: ["3.11"] only by default

Net effect on PRs: eliminates CI / Python CI duplicates and drops Gate from 2 runtimes to 1, which should bring PR time back down substantially.

[agents-workflows-bot]

⚠️ Action Required: Unable to determine source issue for PR #125. The PR title, branch name, or body must contain the issue number (e.g. #123, branch: issue-123, or the hidden marker ).

[Copilot]

Pull request overview

This PR optimizes CI workflow execution for pull requests by eliminating redundant test runs and reducing the test matrix for PR validation.

Changes:

• Removes pull_request trigger from CI workflow, making it run only on pushes to main
• Reduces Gate workflow Python version matrix from ["3.11", "3.12"] to ["3.11"] for faster PR validation

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/pr-00-gate.yml	Reduces Python test matrix to 3.11 only for faster PR validation
.github/workflows/ci.yml	Removes pull_request trigger to eliminate duplicate CI runs on PRs

[stranske-keepalive]

🤖 Keepalive Loop Status

PR #125 | Agent: Codex | Iteration 0/5

Current State

Metric	Value
Iteration progress	[----------] 0/5
Action	wait (missing-agent-label)
Disposition	skipped (transient)
Gate	success
Tasks	0/2 complete
Timeout	45 min (default)
Timeout usage	11m elapsed (25%, 34m remaining)
Keepalive	❌ disabled
Autofix	❌ disabled

🔍 Failure Classification

| Error type | infrastructure |
| Error category | resource |
| Suggested recovery | Confirm the referenced resource exists (repo, PR, branch, workflow, or file). |