docs: split architecture into owner documents and tighten agent guidance

[joshistoast]

Summary

Agent guidance was long, partly stale, and routed every ownership change through a 56 KB architecture doc. This PR:

• Moves per-feature behavior specs out of invokeai/frontend/webv2/ARCHITECTURE.md (56 KB → 18 KB) into owner READMEs, listed in a new "Owner documents" table, and adds short AGENTS.md pointers next to the Canvas, projects, and extensions docs.
• Fixes stale references: removed workbench/backend and migrated ownership rows, deleted test files cited by the Canvas doc, and paths with no stated root.
• Adds invokeai/frontend/webv2/performance/README.md: what each performance gate checks, causes and fixes per failure message, byte-budget rules, and remediation tickets.
• Tightens all AGENTS.md files and owner specs into shorter sentences without dropping conditions; adds rules for delegation (every file edit goes through an implementation subagent), stable review candidates, shared-resource coordination, and repo guidance overriding external skill defaults.
• Adds pnpm dev:mock (Vite against the in-repo mock backend on a free port), a shared dragWithMouse test helper for dnd-kit droppables enabled after drag start, and performance gate failures that name the counted specifier, the fix, and the guide.
• Replaces pnpm -C <package> from the root with running pnpm in the package directory (Corepack resolves pnpm per directory) in docs and two Makefile targets.

QA Instructions

From invokeai/frontend/webv2:

• pnpm lint: passed.
• pnpm test:browser src/features/gallery/ui/GalleryBoardsPanel.browser.test.tsx: 25/25 with the shared drag helper; a single-jump variant fails 3 drag tests.
• pnpm check:release: test:browser had 2 failures under load (WorkbenchHotkeyRuntime.modal, WorkflowFieldInput), both untouched by this PR; each passed 81/81 twice when rerun alone. The remaining gates then ran and passed: test:fixtures, test:performance:architecture, test:project-files:browser, test:accessibility:browser.
• Gate messages: with the @platform/ui importer budget temporarily lowered, the build gate reports platformUiBarrel has 160 direct importers of @platform/ui (budget 150), the subpath remedy, and Causes and fixes: invokeai/frontend/webv2/performance/README.md. A forced editor-runtime hit reports the same guide line.
• pnpm dev:mock --port 5373 --strictPort: page and proxied /api return 200; SIGTERM exits 143 and SIGKILL of Vite exits 137, releasing both ports.
• Docs: oxfmt --check clean; every relative link resolves; every path resolves from each doc's stated root.

The later doc-only commits ran formatting and link checks, not application suites.

Review

Resolved findings: wrong default Vite port (5174), editor-runtime failures missing the guide pointer, a Canvas doc sentence overstating the dependency rule, remaining pnpm -C docs, dev:mock reporting exit 0 on signal death, and wording that changed meaning while tightening (authentication dropped from "auth", "modify" widened to "touch", lock-order example read as the full rule, requirements read as facts). The extensions README no longer claims header menus render menu contributions; nothing reads registered menus yet.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Meaningful regression coverage added / updated where needed; obsolete tests/code removed
• Persisted-state and API changes include required migrations / compatibility validation
• Relevant performance/efficiency opportunities considered; material claims have evidence
• Material review findings resolved and relevant checks rerun
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

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[JPPhoto]

Suggested change:

• invokeai/frontend/webv2/src/workbench/canvas-engine/ARCHITECTURE.md:209: The documented @vitest/browser-playwright version is 4.1.9; package and lockfile use 5.0.0. This mismatch also exists in the base. Update the version separately.