feat: add initial contributor roster and validation tooling

[Piinks]

Based off of #4 - should be reviewed first

Part 3 of 4

• Part 1: chore: add linter rules, sync bot list, and clean activity dataset #4
• Part 2: docs: add ladder governance, issue templates, and legacy AUTHORS archive #5
• Part 3: this
• Part 4: feat: add quarterly audit CLI, grace period automation, workflows, and agent skill #7

Full Picture: #3

Towards flutter/flutter#186415
RFC: flutter/rfc#36

Seeds the initial Flutter Contributor Ladder roster and adds validation and generation tooling:

• Adds data/contributors.yaml as the single source of truth for non-Maintainer ladder tiers (Committer, Reviewer, Emeritus) and generates ROSTER.md and EMERITUS.md.
• Adds tool/validate_roster.dart and tool/generate_roster.dart (with --check mode) to validate schema integrity, bot/maintainer exclusions, and markdown synchronization.
• Adds tool/seed_initial_data.dart and unit tests in test/roster_validation_test.dart, and wires roster validation into .github/workflows/validate.yml.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[zanderso]

I think the PRs might not be split up quite right? It looks like there's some overlap between this PR and number one in the sequence?

[zanderso]

code needs a copyright header. here and below.

[Piinks]

Done!

[Piinks]

I think the PRs might not be split up quite right? It looks like there's some overlap between this PR and number one in the sequence?

Yes, from the PR description: Based off of #4 - should be reviewed first
I will merge that and get this rebased. The next one is also based off this one, so let me resolve the license headers (nice catch!) before reviewing that one so this can land. Alas, stacked PRs would not work.

[jtmcdole]

lgtm, but waiting on Zach's change request