fix: regenerate catalog with the built-in GitHub token

[chocholous]

Catalog regeneration currently fails after merges because it requires an unconfigured GitHub App. Use the built-in GITHUB_TOKEN with contents:write only in the regenerate job; other jobs retain contents:read. Remove App credentials and App-specific identity lookup, and use the standard GitHub Actions bot identity. Keep strict validation, serialization and push retries. Include the workflow itself in the push path filter so merging this fix triggers regeneration, and explicitly target main when pushing.

Verified in apify/awesome-skills on an isolated branch with the same generation/authentication steps and branch-specific targets:

• First run regenerated and pushed exactly marketplace.json, agents/AGENTS.md and the README skills table: 18 stale entries restored to 20; all three remote files match the trusted generated baseline byte for byte.
• Rerunning checked out the latest branch, reported a fixed point, and created no additional commit. The bot push did not create a recursive workflow run.
• Main remained unchanged throughout. No repository settings, App installation or PAT were needed.

Functional test, two successful attempts.

The main-branch push has not been exercised by this test; it becomes the production verification after this PR is merged. Current main reports no branch protection or applicable rulesets.