ci(pr-proof): treat top-level repo metadata as non-runtime (#1647)

* ci(pr-proof): treat top-level repo metadata as non-runtime

The gate refused a PR whose only extra file was .gitignore, because an
unlisted path counts as runtime. That is the allowlist working as
designed, but .gitignore cannot change what an installed CLI or broker
does, so it is a genuine gap rather than a gate defect.

.gitignore, .gitattributes and .editorconfig are now listed. They are
listed individually rather than as a dotfile glob on purpose: a
top-level dotfile that DOES affect what gets built or installed — an
.npmrc or .nvmrc — must keep demanding a proof. A glob would have
exempted those too.

Tests: 87 passing, 6 skipped. Two mutations checked — removing the
entries turns the metadata test red, and widening them to a dotfile glob
turns the .npmrc guard red.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014N3p9VEngj9kLDFFhrzHNd

Session-Id: 246fba6e-6436-46ae-bc50-6bb3cca0d95e

* ci(pr-proof): drop .gitattributes from the metadata exemption

Review pushed back on exempting .gitignore, and checking the claim
properly turned up a sharper problem with a neighbour I had added by
association.

.gitattributes is now NOT exempt. `export-ignore` removes files from
`git archive`, and .github/workflows/relayflow-pr-proof-broker.yml
builds its isolated source tree with exactly that command, so a
.gitattributes edit can change what a proof compiles against.
`text`/`eol` normalisation can also change file contents. That is a
different class from "which files git tracks" and I grouped it in
without thinking it through.

.gitignore stays, for reasons that are checkable rather than assumed:

- every published package declares an explicit `files` array, so npm
  never falls back to .gitignore for packaging
- the shipped consumer of a .gitignore, buildIgnoreMatcher in
  packages/cloud/src/workflows.ts, reads the USER's project root, not
  this repo's, so editing this file cannot change what an installed
  agent-relay does in someone else's project

Its only remaining reach is this repo's own cloud tarball, which is the
same CI scope already exempted through workflows/ and .github/.

Tests: 88 passing, 6 skipped, including a new guard asserting
.gitattributes is treated as runtime.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014N3p9VEngj9kLDFFhrzHNd

Session-Id: 246fba6e-6436-46ae-bc50-6bb3cca0d95e

---------

Co-authored-by: Proactive Runtime Bot <agent@agent-relay.com>