ci: migrate GCC post-commit CI ownership and operations to RISE

[pz9115]

Summary

Move the code-side ownership and operation of GCC post-commit CI from the former personal integrations to riseproject-dev.

This PR migrates the toolchain dependency, artifacts, status issues, dashboard, workflow identities, runner policy, and operational documentation while preserving the existing dashboard history.

What changed

Repository and toolchain ownership

• Point the riscv-gnu-toolchain submodule at riseproject-dev/riscv-gnu-toolchain-ci.
• Track build-frequent and pin the reviewed toolchain handover commit.
• Replace active personal repository, Pages, Actions, artifact, and issue references with RISE-owned or repository-relative values.
• Add a migration guard that prevents legacy service identities and retired GCC 14 configuration from returning.

Artifacts and status reporting

• Make artifact sources explicit instead of relying on hard-coded personal repositories.
• Read comparison artifacts, build logs, baselines, and duplicate issues from the current RISE repository.
• Generate run and issue links from github.server_url and github.repository.
• Prevent failed or invalid status issues from being selected as valid baselines.

Dashboard

• Read status issues and artifacts from riseproject-dev/gcc-postcommit-ci.
• Publish links under https://riseproject-dev.github.io/gcc-postcommit-ci/.
• Paginate issue ingestion and accept only supported dashboard status-title formats.
• Ignore pull requests, malformed titles, duplicate hashes, and missing or expired artifacts.
• Stop resetting the toolchain submodule to an unreviewed remote head during deployment.
• Add unit coverage for pagination, filtering, deduplication, and empty-repository bootstrap behavior.

Generated dashboard HTML, CSV data, and testsuite history are deliberately not regenerated or included in this PR.

Workflow hardening

• Declare workflow permissions and reusable-workflow secrets explicitly.
• Restrict persistent build jobs to dedicated runners labeled self-hosted, linux, x64, and rise-gcc-ci.
• Keep external-fork pull requests off persistent self-hosted runners.
• Correct workflow inputs, outputs, retry IDs, artifact fallback behavior, and manual-dispatch configuration.
• Move the vendor-option checkout from the retired GCC 14 branch to GCC 15.
• Retain the maintained GCC 15 and GCC 16 release conventions.

Operations

• Add deployment, cutover, rollback, governance, and runner guidance.
• Add a maintenance runbook covering regression triage, infrastructure failures, allowlists, caches, dashboard recovery, and required labels.