Phase 3: remove legacy dual paths and release v3.0.0

[olliethedev]

Context

Final phase of the v3.0.0 core simplification, after all plugin sweeps in #136 land. v3.0.0 is not yet published to npm, so all breaking removals batch into this release. This PR should be reviewable as "deletions only" — no migrations happen here.

During phases 1–2 the old and new paths coexist so main stays green; this issue deletes the old paths.

Deletion checklist

Provider / context:

• Remove Link / navigate / refresh / Image / apiBaseURL / apiBasePath fields from all plugin override types (top-level router + api from Top-level router adapter on StackProvider (framework presets for Link/navigate/refresh/Image) #127 become the only path)
• Remove the per-plugin override precedence / fallback branches in StackProvider
• Remove onBefore*PageRendered guard callback types and their hook shims (auth provider route gating from Auth provider contract: centralized identity + permissions (getIdentity/can) across client and backend #128 becomes the only path)
• Remove manual identity props (currentUserId, headers, loginHref) from CommentThread and any other components that took them (identity comes from Auth provider contract: centralized identity + permissions (getIdentity/can) across client and backend #128's provider)

Templates / examples:

• Codegen templates (scripts/codegen/files/*) use only the new setup (entry factories Framework entry factories: collapse page handler and API route setup to one-liners #130, top-level providers); delete old-style layout/page-handler templates
• Playground and docs examples updated to new-only

Tests:

• Delete E2E/unit tests that exercise removed paths; verify new-path coverage is complete across all three frameworks

Docs:

• Rewrite installation guide against factories/providers only (manual glue moves to an "advanced" appendix or is dropped)
• Update all plugin MDX pages (override tables, auth examples)
• Write v2 → v3 migration guide in docs/content/docs/breaking-changes.mdx covering: yar route context (already documented), removed override fields, removed guard callbacks, removed identity props, new provider setup
• Update agent skills (btst-client-plugin-dev, btst-integration, btst-build-config) to new patterns

Verification / release:

• rg sweep: no references to removed fields/props/callbacks anywhere in the monorepo
• pnpm build && pnpm typecheck && pnpm lint && pnpm test green; all three codegen E2E suites green
• Publish @btst/stack 3.0.0 to npm (gate: everything above complete)

Depends on

• Phase 2: per-plugin adoption sweep of the new core primitives #136 (all plugin sweeps merged)
• All phase-1 core issues closed: Auth provider contract: centralized identity + permissions (getIdentity/can) across client and backend #128, Internal resource-hook factory: deduplicate query-keys/hooks boilerplate across plugins #129, Notification provider: pluggable notify() instead of hardcoded sonner toasts in every plugin #131, i18n provider: translatable plugin UI strings with English defaults #132, URL-synced list state: filters, tabs, and pagination survive refresh, back button, and sharing #133