Add automated proposal quality gates

Adding three GitHub Actions workflows to automate initial quality checks on incoming Dev Fund proposals. These run on PR open/reopen and handle checks that don't need human review time.

1. Champion Requirement Check (champion-check.yml)

Checks whether the proposal has a valid Champion/Endorser field.

• If champion is present: adds champion-confirmed label and posts a confirmation comment.
• If champion is missing or TBD then posts a comment explaining the requirement, adds needs-champion label, and closes the PR

1. AI Signal Analysis (aipricheck.yml)

Runs pattern-matching checks for common AI-generated content signals and posts results to private Slack channel. Nothing is posted on the PR itself. Checks for:

• Made-up Canton/Daml terminology (sub-net, Loop node, Daml VM, Canton IdP, sharding, etc.)
• Broken hyphens from AI copy-paste (word- space, space -word patterns)
• Em-dash overuse
• Balanced pros/cons comparison tables
• Over-structured headers
• Notes missing sections (GTM/Adoption, Maintenance, Acceptance Criteria) as non-scored info.

Outputs a risk score (LOW/MEDIUM/HIGH) to Slack only. Requires SLACK_WEBHOOK_URL to be added into repo secrets.

1. SIG Label Check (sig-label-check.yml)

Checks whether the PR has at least one SI label so the right domain experts can find and review it.

• If SIG label is missing posts a comment listing all available SIG labels and adds needs-sig-label
• If SIG label is added later: removes needs-sig-label on re-trigger

Does not close the PR

Requires Labels to create in repo settings:

• needs-champion
• champion-confirmed
• needs-sig-label