Choose the sandbox backend at install time so Add node is ready after install

[SaladDay]

Summary

Phase 2b-installer of the new-user install work: the Core installer picks the sandbox backend at install time, so Add node is available straight after install. The installer no longer adds its own host as a node.

• install.sh --sandbox docker|microsandbox|e2b|none
  • The default is docker. --web-only allows only none.
  • After the services are healthy, the installer creates the deployment once through POST /core/v1/sandbox/deployment with the Core key. It sends the same body Web's setup sends.
  • Docker and microsandbox use the bundled copy of apps/web/src/features/sandbox/standard-sizes.json, the single source for the Standard size, plus the bundle manifest's Runtime release.
  • E2B takes --e2b-api-key-file and --e2b-template. The key file must be an absolute path, private, not a symlink and at most 4 KiB; it is read with O_NOFOLLOW and fstat. E2B is refused before anything is created when the public URL is loopback or missing.
  • The key is sent only in the request body and is redacted from Core's error text.
  • The choice is a one-time action: it is not stored as a setting, and repair or --convert never create a deployment.
  • If Core refuses, the services keep running, the installer prints Core's message and exits non-zero. The same happens after a failed first start: the message then says the backend was not chosen and names where to choose it.
• Retired:
  • --sandbox-provider and --provider fail with a message that names --sandbox.
  • local_node.py becomes sandbox_setup.py. The installer's own node enrollment and its use of PARSAR_NODE_ENROLLMENT_TOKEN are removed.
  • Hosts, including the Core host, are added with Add node in Web.
• Build: build-core-distribution.sh copies the sizes file into the bundle, and bundle verification requires it.
• Docs: README, the getting-started guides, configuration.md (the sizes table now points to the file), the Hosted Sandbox Manager guide, CONTRIBUTING and the microsandbox README.

Review

• Independent blind review: no P0 or P1. It verified that the POST bodies match Web's and Core's input, that the E2B key never leaks, that refusals leave nothing behind, and that parsar works without local_node.py.
• The P2 (an explicit --sandbox choice lost after a failed first start) and the P3s were fixed in 92cd1480: stale docs, tests for key-file refusals and a missing sizes file, O_NOFOLLOW reads, and exact size keys. They were verified by tests and the gate, per the small-fix rule.

Verification

• Full local server gate on 92cd1480, the exact head including main 1f0920aa, passed. The Playwright browser cases were skipped on the server (Chrome unavailable; approved skip).
• Real check on an isolated mx1 install at ac05d3aa (Kimi only):
  • The default install created a Docker deployment matching the bundled sizes and manifest. Web saw a configured provider with zero nodes, so the Nodes page offers Add node.
  • --sandbox none created no deployment.
  • Loopback E2B, the retired flags and a web-only backend were refused with nothing created.
  • A node added with Web's current command came online, and a real Kimi Turn ran on it.
• E2B and microsandbox selection are unit-tested only. Real checks of both are scheduled for the final acceptance.

---

^{Need help on this PR? Tag @codesmith with what you need. Autofix is disabled.}