persist AI provider connections while providers.json stays executable

[atomantic]

Summary

• Adds the durable half of the provider connection graph from the design record, on top of the read-only preview Preview provider connection identity and harness bindings without changing execution #6366 shipped. Three db-primary tables (ai_connections, ai_harness_bindings, ai_route_bindings) record which backend each harness talks to and which executable route it projects.
• data/providers.json stays the execution contract and stays fully materialized, so a downgraded release runs unchanged with the tables idle — and every saved selection, task pin, fallback reference and activeProvider keeps naming the provider id it always did.
• One reconciliation pass serves boot, every legacy provider write and a re-upgrade after a downgrade, so those paths cannot drift apart. It acknowledges a projection whose file write landed, retries one that did not, and refuses to overwrite a third value another writer put there — keeping the route's last valid executable settings readable and blocking further graph edits to that binding until repaired.
• Deletions are not resurrected. An edit that moves a binding's backend clones a shared connection (rather than repointing another harness), updates an exclusive one in place, and splits a binding whose CLI/TUI siblings now disagree.
• The vendored toolkit gains one injected extension point (onProvidersSaved) plus applyProviderPatches, so it stays self-contained while PortOS learns about every write to the file it owns.
• New endpoints: GET /api/providers/management, revision-checked link / link/preview / unlink on a binding, and DELETE for an emptied connection. The flat GET /api/providers and every existing mutation are untouched; an install without a database gets an explicit PROVIDER_GRAPH_UNAVAILABLE rather than a silently empty graph.
• Migration 358 is gated on its INPUT and parks an untouched pre-graph copy of the install's own providers.json; that derived path is declared in migrationOwnedPaths.js and ships no seed.
• Graph rows never federate — endpoints, credentials and projection snapshots are machine-local, with a guard test mirroring privacyNeverFederates.test.js.

Linking UI is out of scope (#6369); this PR ships the service validation and projection contracts it will call.

Test plan

• cd server && npm test — 2017 files, 40237 tests green.
• cd client && npm test — 872 files, 10612 tests green.
• npm run test:db — 34 files, 291 tests green against portos_test, including the new providerGraphStore.db.test.js (partial unique indexes, (binding_id, mode) uniqueness, FK refusal, JSONB snapshot round-trip, transactional rollback).
• New suites: server/lib/providerGraphRecords.test.js (import round-trip fidelity, DTO sanitization, crash recovery incl. the refuse-a-third-value case, detach/update/split, partial-restore in both directions), server/services/providerGraph.test.js (stage → write → acknowledge ordering, re-entrancy latch, revision gate), server/routes/providers.graph.test.js, server/lib/aiToolkit/providers.hostPersistence.test.js, scripts/migrations/358-provider-connection-graph.test.js, server/services/sharing/providerGraphNeverFederates.test.js.
• Local ollama reviewer returned inconclusive (no model configured for the ollama reviewer on this install); it is the optional reviewer, so no round was applied.

Closes #6367