[MAINTENANCE] Surface the RFC threshold in the PR template, AGENTS.md, and a new check

[joshua-stauffer]

Summary

@abdullah094 opened #12012 adding Exasol support and got told an RFC was required after writing the code. That's on us — the RFC criteria live in CONTRIBUTING.md, and nothing in the pull-request path pointed at them. I said on that PR I'd fix it for the future; this is that fix.

What changed

1. Pull-request template — states the RFC threshold inline (in a comment above the checklist, so the criteria are visible while writing the description) and adds a checklist item for it.

Two drive-by fixes in the same file:

• The AGENTS.md link was relative, so it resolved to .github/AGENTS.md (404) rather than the file at the repo root. Now absolute.
• Remaining github.com/great-expectations/... links normalized to fivetran/..., matching CONTRIBUTING.md and the rest of .github/.

2. AGENTS.md gains a "Before opening a pull request" section. This is the part that matters for tooling. gh pr create --body discards the pull-request template entirely, and nothing injects it into a coding agent's context — so a template-only fix reaches humans opening PRs in the web UI and misses every programmatically-opened PR. AGENTS.md is the file agents read from the repo root by convention, which makes it the highest-leverage place to put this.

3. New pr-hygiene check (.github/workflows/pr-hygiene.yml).

Flags changes that look like new backend support and asks for one line in the description — either RFC: <link> or No RFC needed: <reason>. Signals, all added-file only, so modifying an existing datasource doesn't trip it:

Signal	Means
new great_expectations/compatibility/*.py	new optional third-party dependency
new great_expectations/datasource/fluent/*_datasource.py	new fluent datasource
new reqs/requirements-dev-*.txt	new backend requirements

Design notes:

• It enforces that the question was answered, never what the answer is. A false positive costs one line in the description, not a blocked PR. That keeps it useful without making it adversarial.
• When it trips, it posts a comment explaining what to do (once, marker-guarded against re-posting). A failing check annotation alone doesn't carry enough context for a first-time contributor — and a prescriptive failure message is also what lets a coding agent self-correct in one iteration.
• pull_request_target, metadata-only. It reads PR metadata through the API and never checks out or executes PR code, so it's safe on fork PRs. That trigger is the point: it fires the moment a PR opens, rather than waiting for a maintainer to approve a CI run — which is exactly the delay that made [FEATURE] Exasol integration #12012 go wrong.
• It also checks the title prefix, exempting bot authors whose titles are generated upstream.

4. Removes the legacy Markdown issue templates. bug_report.md and feature_request.md were superseded by the structured issue forms but never deleted, so GitHub kept offering all four in the chooser. Because the Markdown templates carry labels: '', issues filed through them arrived with neither a type label nor triage — silently bypassing the triage queue that the ready-for-work claiming gate depends on.

Verification

• pre-commit run --files ... — all hooks pass (check-yaml, prettier, end-of-file-fixer, trailing-whitespace)
• Workflow YAML parses; embedded script passes node --check
• Signal regexes and the body scanner unit-tested against 19 cases, including the real [FEATURE] Exasol integration #12012 file list (trips all three signals) and the unedited template (correctly does not pass — the scanner strips HTML comments first, otherwise the template's own guidance text would satisfy the check)
• ci/checks/check_repo_root_size.sh passes (no root files added)

Not done here

• pr-hygiene is not yet a required check. It's a separate workflow, so it can't join ci-required's needs: list — that would also put it behind the actor-permission gate and destroy the early-feedback property. Making it blocking is a branch-protection setting; I'd suggest living with it advisory for a few weeks first to see the false-positive rate.
• It won't run on this PR. pull_request_target uses the workflow from the base branch, so it goes live on the first PR opened after merge.

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