feat(web): new-workbench picker deploys chosen bench agents into the room (CL-8442)

[TheGreatAxios]

Summary

• apps/web/src/git-fetch.ts: fetches one file out of a hub asset repo from the browser (isomorphic-git's upload-pack wire over the asset's smart-HTTP git remote, with a short-lived read-only token) — the read-side counterpart to git-push.ts.
• apps/web/src/agent-source-read.ts: readAgentSource(tenantId, assetId, assetName) mints a can_read git token, fetches the workflow asset's main, and parses its workflow.js entry (parseWorkflowSourceEntry) back into { systemPrompt, declaredSources }.
• apps/web/src/chat/threads-api.ts: ChatAgent now carries assetName (the raw workflow asset name, vs. name's display alias for Myra), and exports isMyraAgent so pickers can exclude her.
• apps/web/src/pages/new-workbench-picker.tsx: restores the agent multi-select — lists the bench tenant's non-Myra chat agents via listChatAgents, lets the person check which ones join the room.
• apps/web/src/workbench-create.ts: createWorkbench takes pickedAgents; for each, reads its source off the bench tenant and re-deploys it into the child tenant via deployAgentSource (same path Myra's own deploy uses — a child's deploy route rejects the parent's inherited asset).

Upstream gaps

• No stock file-read route exists for a workflow-kind asset (only package-registry tarballs get one — see vendor/intx/hub-api/src/routes/assets.ts's /:assetId/tarballs route). Reading an agent's source therefore goes over the smart-HTTP git remote with a read-only token rather than a simple GET; flagged here per "upstream asks go to user," not filed anywhere.

Test plan

• bunx tsc -p apps/web --noEmit — clean
• bun run fmt / bun run lint — clean (pre-existing sidecar warnings only, unrelated)
• bun test apps/web/src/workbench-create.test.ts apps/web/src/agent-deploy.test.ts apps/web/src/chat — no new failures (pre-existing jsdom-less failures in markdown.test.tsx / tool-activity tests are unrelated to this change)

[linear-code]

CL-8442