Bring the agent-core changes of desktop 0.0.1 to main

[sosidudku1]

The agent-core (src/) files that still differ between main and rel/xp-build, taken as they are in the desktop 0.0.1 build (rel/xp-build @ a283d48). After this PR, src/ on main is the same as on the desktop branch, so the terminal and the desktop app run the same agent.

#595 left these on rel/xp-build because the HTTP extensions they build on lived only there. This PR brings the extensions and everything built on them together.

What changes

• HTTP extensions the desktop app talks to: coding mode over HTTP (route-coding-mode), the context preview before the first message (route-context-preview), the per-request context and the route table entries for both. Session rows carry their title.
• SSE frames:
  • error / provider_waiting name the billing cause and the links that failed before;
  • a stopped turn's loop_failed frame reads as cancelled;
  • Fusion worker progress is a named frame;
  • a reply that never streamed still reaches a streaming client.
• fix(http): keep idle SSE streams alive and stop replaying dead approvals #607: idle SSE streams get a : keepalive comment every 15 s, and dead approval requests are no longer replayed. If fix(http): keep idle SSE streams alive and stop replaying dead approvals #607 is merged first, this PR's diff shrinks by those files; the result is the same.
• Runtime: previewPrompt in bootstrap.ts builds (never runs or stores) the prompt the next turn would open with, for the context preview; the approval gate answers isPending(id) for a host that mirrors requests.
• Tests: frame tests for cancel, failure, Fusion worker and reply; provider-waiting stream; coding mode; context preview; the fallback billing integration test.

Nothing outside src/ changes. The TUI does not call the new routes. They are only reached by a client of atomic-agent serve, such as the desktop app.

Checks

• On the MacBook Pro, with this branch: agent lint and build clean; the whole vitest suite 12 515 passed, 4 skipped, 1 failed. The failure is the known EACCES in local-models-orchestrator-auto-update.test.ts, which also fails on a clean main.
• The same files are what the desktop 0.0.1 build runs. It passed the full desktop smoke (1214 passed; the 14 failures are the known bench ones), the e2e scenarios (8/8), and a signed 0.0.90 → 0.0.91 app update on macOS.