Fold the Stream Deck plugin's design rationale in-tree before the pre-migration repo is archived

[amrmelsayed]

Context

From the pr-gate integration review of PR #1387 (claude lane, non-blocking). apps/streamdeck/README.md defers architecture and design decisions to the pre-migration repository's PLAN.md, which #1347's plan deliberately did not import. That is a live documentation dependency on a repository whose retirement is already a tracked follow-up of the migration.

Scope

Move the still-relevant design rationale in-tree — either a Design section in apps/streamdeck/README.md or an apps/streamdeck subsection of codev/resources/arch.md (implementer's call) — and update the README pointer so provenance stays in the History section only.

Sequencing

Must land before the pre-migration repository is archived (that retirement is gated on the next release / the sdk's first npm publish). Natural bundle-mate for the retirement work itself.

[amrmelsayed]

Shipping record: PR #1452 merged 2026-08-15T02:20:06Z (7/7 green). The plugin's design rationale now lives in a ## Design section of apps/streamdeck/README.md, arguing four decisions from their consequences — stateless controller, canonical verbs over the command relay, one shared selection, keys-commit/dials-review. The pointer to an external planning document is gone and provenance reads in codev's own terms in History (imported under #1347 at 77be3d0; retirement of the pre-migration repository gated on the sdk's first npm publish).

This closes the documentation dependency that blocked that retirement, which was the whole point of the issue.

Review note: this lane ran without a CMAP pass (the builder skipped it; I ratified that at review time rather than retroactively — reasoning on the PR). The compensating check was verifying each load-bearing claim against the code, since a reconstructed rationale fails by drifting from the system it describes rather than by reading badly. All three checked out, and the exercise turned up a stale comment in the VSCode relay as a by-product — now #1454.

No release-notes entry, deliberately: this is internal documentation with no user-visible behaviour change, and release notes are user-facing.