add triage cadence, roles & escalation matrix for stale-issue automation

[IanoNjuguna]

Description

Related to #274

New issues and PRs in this repo currently have no documented triage process. Contributors don't know what response to expect, and maintainers have no shared cadence or label conventions. This PR establishes one byt making the following additions:

1. TRIAGE.md - defines the triage cadence (weekly triage ahead of each DevEx WG session, 48h PR review SLA, monthly stale revisit), a triage checklist for maintainers, the label taxonomy, response-time expectations, and roles/escalation (disagreements resolved in the weekly WG session).

2. stale.yml workflow - automates the inactivity policy defined in TRIAGE.md: issues go stale after 30 days (closed 14 days later), PRs after 21 days (closed 7 days later); pinned and security labels are exempt; any activity resets the clock.

3. CONTRIBUTING.md - links the new triage doc from the submission process so contributors see expectations at the point of contributing.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality — stale-issue automation)
• Breaking change
• Documentation update
• Refactor
• Other (please describe):

Changes Made

• TRIAGE.md (new, 60 lines) — triage cadence table, maintainer triage checklist, label definitions (triage, needs-info, good first issue, help wanted, stale, pinned), SLAs (1-week issue response, 48h PR review), stale policy, roles & escalation.
• .github/workflows/stale.yml (new, 43 lines) — actions/stale@v9 on a daily cron (06:17 UTC, off the top of the hour) plus workflow_dispatch; least-privilege permissions (issues: write, pull-requests: write only); messages point contributors back to TRIAGE.md.
• CONTRIBUTING.md (+2 lines) — one-paragraph pointer to TRIAGE.md in the submission-process section.

No site code, docs content, or existing workflows are touched.

Checklist

• My code follows the project's coding style and guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation (if applicable).
• My changes generate no new warnings or errors.
• I have added tests that prove my fix is effective or that my feature works. (N/A — docs + scheduled workflow; workflow supports workflow_dispatch for a manual dry-run)
• I have checked for merge conflicts.
• I have rebased my branch onto the latest main or master branch. (will rebase before merge if needed)

Follow Up

The security exemption requires the label to exist — check the repo's label list has security, pinned, stale, triage, needs-info (the stale action creates stale itself, but the exemptions silently no-op if missing).

First run is manual-friendly — you can trigger it via workflow_dispatch right after merge to confirm behavior before the cron kicks in; with no debug-only run it will start labeling real issues, so give the WG a heads-up first.