fix(cli,mcp): keep saved token on transient errors, harden API error handling

[bmc08gt]

blip logs a user out whenever /v1/sessions/me returns anything other than 200. ensureToken in cli/.../client/ApiClient.kt treated a 5xx, a 429, or a network error the same as an invalid token: it created a new anonymous session and overwrote ~/.config/blip/config.json. A paying user hitting a deploy restart lost their saved token and their Pro inboxes.

CLI

• ensureToken discards the saved token only on 401. Other failures throw "Blip API unavailable, try again" and leave the token alone. A failed POST /v1/sessions now errors instead of saving nothing.
• config.json is created rw------- and ~/.config/blip rwx------ (skipped on non-POSIX). It held the token with default umask permissions.
• Config.load() used to swallow parse errors and return an empty config, which then minted a new session. It now prints the file path and the parse error and exits 1.
• blip open still passes the token in the /app?token= URL. The web app strips it on load; a one-time code exchange would avoid it entirely and is left as a follow-up. Comments mark both URL-token call sites.

MCP server

• blipFetch maps errors to messages an agent can act on: 401 → invalid or expired key; 402 → upgrade hint; 403 → upgrade hint when the server message mentions plan/tier, otherwise "Access denied" (the server also uses 403 for resources the key doesn't own); 429 → rate-limit message with the Retry-After value.
• wait_for_email retries 5xx, 429 and network errors with backoff from 2s to 10s (or Retry-After if larger) instead of failing the tool call. The overall timeout is unchanged.
• zod was imported but only installed transitively through the SDK. It is now a direct dependency (~4.3.6, what the lock already resolved), and the SDK is pinned to ~1.27.1.
• The reported server version is read from package.json; it was hardcoded 0.1.2 against package version 0.1.3.

ttl_minutes is sent as windowMinutes, which matches CreateInboxRequest: InboxService.createInbox uses it as the TTL for AGENT (capped at 90 days) and as the sniper window for PRO. No change there.