docs: add 2026-08-13 roadmap plan

[aaearon]

Adds docs/plan-2026-08-13-roadmap.md, the planning document that drove PRs #47-#55.

It covers five items, sequenced 3 -> 1 -> 2 -> 4 -> 5:

1. Item 3 - stale SDK profile paths + retroactive CHANGELOG note (fix: correct stale SDK profile path in grant configure #48)
2. Item 1 - 429 / transient auto-retry safety on non-idempotent POSTs (chore: disable SDK transient retry on SCA and UAR clients #50), which gated the dependency upgrade
3. Item 2 - replace the abandoned rhysd/go-github-selfupdate to clear GO-2026-5932 (fix: replace abandoned go-github-selfupdate to remove openpgp CVE #51)
4. Item 4 - GCP support for list, elevate, status and revoke (feat: GCP support for list, elevate, status and revoke #53)
5. Item 5 - grant k8s cluster listing, kubeconfig, elevation and exec-credential

Item 5 is paused: it is by far the largest item and importing the SDK's k8s package adds 16 modules to the build graph. The work in progress lives in draft PR #55.

Documentation only - no code changes.

[Copilot]

Pull request overview

Adds a dated roadmap/planning document to docs/ capturing the investigation notes, decisions, risks, and acceptance criteria that drove the stacked PR sequence (#47–#55), with explicit references to relevant code locations and SDK behavior.

Changes:

• Add docs/plan-2026-08-13-roadmap.md documenting execution order, decision records (D-1/D-2/D-4/D-5), and test/verification plans for items 1–5.
• Capture key risk/acceptance criteria notes for retry policy, self-update replacement, GCP support, and the paused k8s work.

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