ci(e2e): skip e2e on docs and version-bump-only PRs

[ben-dz]

Summary of Changes

• Skip the e2e and shreds-e2e suites on PRs where the suites add no signal — version bumps and docs/RFC-only changes — so they don't burn ~20 minutes of self-hosted runner time building images and running the full suite.
• A new gate step in each workflow's setup job inspects the PR's changed files via the API. A PR qualifies only if every file is either markdown (*.md, any path; renames/copies must come from markdown too, since a rename also deletes the source path) or Cargo.toml/Cargo.lock with diffs touching nothing but version = "..." lines, with the two Cargo files changing together. A dependency bump fails the check — cargo update changes Cargo.lock checksum/source lines, an inline requirement change in Cargo.toml doesn't match the version-line shape, and a requirement change that leaves the lock untouched fails the pairing rule — as does any code file or an API-truncated patch. All of those fall through to a normal e2e run. Markdown is verified inert: no .md files are used as test fixtures, embedded via include_str!, or read by e2e code.
• Because the gated matrix jobs never create their shard check runs, the gate reports the required e2e (shard 1-5) / shard-e2e (shard 1-4) contexts as successful on the PR head via the checks API (same pattern the trusted fork-PR flow already uses), so branch protection doesn't block the merge.
• The gate runs only on same-repo pull_request events; fork PRs, pushes to main/hotfix, and trusted workflow_dispatch runs are unaffected. Any gate error fails open to running e2e.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Config/build	2	+152 / -0	+152

Workflow-only change; the same gate logic is added to both e2e workflows.

Key files (click to expand)

• .github/workflows/e2e.yml — skip-gate step + early exit in the existing run-e2e gate; reports e2e (shard 1-5) checks when skipping
• .github/workflows/shreds-e2e.yml — same gate; reports shard-e2e (shard 1-4) checks when skipping

Testing Verification

• Ran the gate's file-classification logic (extracted verbatim into a node harness) against real PR data: Bump to 0.27.1 #3874 (0.27.1 version bump) correctly classifies as skippable; client: revert auto-allocate IP on connect when behind NAT (#3861) #3870 (code change) correctly runs e2e.
• Synthetic dependency-bump cases all run e2e: cargo update-shaped Cargo.lock diff (checksum lines); dependency requirement change in Cargo.toml, both inline and multi-line-table form with the lock untouched; lock-only version-line change.
• Synthetic markdown cases: new RFC added, RFC+README edits, markdown deleted, markdown renamed from markdown, and version bump + RFC edit all skip; markdown plus a code file, a file renamed from .rs to .md, and a Cargo.lock with an API-omitted patch all run e2e.
• actionlint passes on both workflows.

[elitegreg]

How about any *.md file?