Point SRE Agent installation docs to Awesome Copilot

[Paul Yuknewicz (paulyuk)]

Summary

Update only canvases/azure-sre-agent/README.md so primary installation uses the merged Awesome Copilot marketplace:

copilot plugin marketplace add github/awesome-copilot
copilot plugin install azure-sre-agent@awesome-copilot

Link the canonical Awesome Copilot installation/discovery landing, and retain microsoft/azure-dev-tools as the actual source repository. The live Awesome Copilot marketplace confirms name awesome-copilot, plugin azure-sre-agent version 0.4.1, and source {repo: microsoft/azure-dev-tools, path: canvases/azure-sre-agent, sha: bc8c3349df123c56eea4a957f8d63d3a4a8fd6e4}. Catalog listing merged in github/awesome-copilot#4595.

This is a default-branch documentation hotfix, not a rerelease. Before and after the README edit, both azure-sre-agent-v0-4-1-84bdf0f and azure-sre-agent-latest resolve remotely to bc8c3349df123c56eea4a957f8d63d3a4a8fd6e4. No refs, versions, manifests, catalog entries, protected receipts, bundles, code, workflows, or release artifacts change. The historical tagged README remains unchanged.

Validation:

• node --test --test-name-pattern='customer installation docs identify|README and documentation image bytes can change without changing a protected tag' test/production-links.test.mjs test/mutable-documentation-release.test.mjs — passed, 2 documentation tests.
• git diff --check and git diff --check origin/main..HEAD — passed.
• git ls-remote origin refs/tags/azure-sre-agent-v0-4-1-84bdf0f refs/tags/azure-sre-agent-latest — passed before and after; both targets unchanged at the SHA above.
• Public raw reads of the live Awesome Copilot marketplace and its README — passed; confirmed marketplace name, source pin, and Install a Plugin discovery heading.
• Runtime/build/package/browser/install validation — not run; not applicable to an installation-text-only edit.

Release and governance checks

• This change contains only content approved for this repository.
• Any required security, legal, licensing, and third-party-notice reviews are complete.
• No internal-only information, credentials, or unapproved payloads are included.
• If this distributes a canvas package, the release process in docs/release-process.md has been followed.

No canvas package is distributed by this PR; the documented documentation-only update policy applies.

Copy/paste team announcement

• Azure SRE Agent 0.4.1: Installation guidance now points to the Awesome Copilot marketplace; the existing released package is unchanged. Rollout owner: Paul Yuknewicz (@paulyuk). Dark-deployed: N/A; documentation-only hotfix.
• Install: copilot plugin marketplace add github/awesome-copilot, then copilot plugin install azure-sre-agent@awesome-copilot; latest canvas-only extension; historical release README; exact version canvas-only extension.