feat(web): Agents and Skills pages read stock deployments; create agent deploys a workflow asset (CL-8433)

[TheGreatAxios]

Summary

• Cuts apps/web over from the deleted /api/tenants/:t/agent-definitions/* and /planner/agent-definitions routes to stock Interchange: an agent is a workflow deployment (GET /workflows/deployments, GET/POST /workflows/runs, .../health, .../events, .../approvals).
• Agents roster (agents-page.tsx) lists deployed definitions with their own top-level run's address, status, and 7-day run count; the per-row model-capabilities fetch and the Archive bulk action are removed (both depended on deleted routes with no stock replacement).
• Agent detail (agent-detail-page.tsx) is now an observe-only page: definition status, its live run's address, health, event log, and approvals — the instructions/model/skills editor is gone (no stock write route backs it).
• New apps/web/src/agent-deploy.ts generalizes myra-deploy.ts's deploy pipeline (ensure a workflow-kind asset, push a rendered source tree over git, deploy via POST /workflows/deployments) over {name, systemPrompt}, resolving the tenant's existing inference offering the same way Myra's own deploy does.
• create-agent-panel.tsx deploys through that path via a useMutation (name + system prompt only — no drafting assist, no per-agent model/skill pins, since the routes that backed those are gone).
• Skills pages (skills-page.tsx, skill-detail-page.tsx, create-skill-dialog.tsx) needed no changes — they already read the native kind:"skill" asset registry via skills-api.ts, never the deleted agent-definition-skill-pin route. agent-skills-picker.tsx (which only backed that pin) is deleted as dead code.
• Data fetching follows TanStack Query throughout: reads are useQuery (useAgentDirectory, useAgentRunHealth/Events/Approvals), the deploy write is a useMutation that invalidates the directory query on success; no useEffect-driven fetches were introduced.

Verified

• grep -rn "agent-definitions\|planner/agent" apps/web/src returns nothing outside apps/web/src/chat/api.ts (owned by a separate ticket).
• bunx tsc -p apps/web --noEmit — clean.
• bun run fmt / bun run lint — clean (pre-existing unrelated warnings in apps/sidecar only).
• Targeted tests (myra-workbench.test.ts, run-status-tone-parity.test.ts, fetch-dedupe.test.tsx) pass; full apps/web suite passes except a pre-existing import.meta.glob bun-test incompatibility in tools/registry-publish.ts (unrelated to this change) and what looks like unrelated test-order flake in composer.test.tsx (passes 24/24 in isolation).

What's left / blockers

• No pagination on the deployments/runs listing (pre-existing PAGE_LIMIT ceiling, unchanged).
• Duplicate/Archive/Move for an existing agent have no stock backing (DELETE /workflows/runs/:runId is workflowRunOperationUnsupported) — dropped rather than shipped as dead buttons.
• chat/api.ts's own agent-definitions/* calls (visible list, capabilities inventory, per-workbench instructions editor) are untouched, per scope — that's a separate ticket's cutover.

Test plan

• bunx tsc -p apps/web --noEmit
• bun run fmt
• bun run lint
• targeted tests pass

[linear-code]

CL-8433