Problem:
========
The GitHub contribution path lacked consistent intake templates, shared
local CI entry points, and automated build, test, formatting, ownership,
and triage feedback. Pull requests could also define workflow changes
that executed before those definitions were merged into trunk.
The selected-actions policy prevented the external Codex action from
running.

Solution:
=========
Add structured issue and pull request templates, CODEOWNERS-based review
routing, path-based area labels, and an expanded contributor guide while
preserving the existing non-code contribution terms.

Add scripts/ci helpers for Ubuntu toolchain setup, Ninja and ccache
builds, clang-format, and the default MTR test selection.

Run automatic PR automation from trunk with pull_request_target.
Verify PR merge parents, disable persisted checkout credentials,
scope permissions per job, restore caches without saving PR-controlled
data, and execute build and MTR helpers from the trusted base SHA.

Build Debug configurations with GCC and Clang, run default MTR with
six-hour limits, publish statuses on the PR head, assign code owners
after OCA verification, reconcile approved PRs daily, and apply PR-only
stale handling.

Change-Id: I75d87674e369b28a7ac6707ebb796e0d2a8bcd83