fix(config): drop --base-url flag and MINIMAX_BASE_URL env

[AndersHsueh]

What

Drop the --base-url global flag and the MINIMAX_BASE_URL env var. baseUrl is now sourced only from config.json's base_url field (or, after OAuth login, the oauth.resource_url field stored alongside it).

Why

An external tool was setting MINIMAX_BASE_URL in the user's shell, expecting mmx to leave it alone. mmx was intercepting it and breaking that tool's own routing. base_url is also fully redundant with mmx config set --key base_url --value <url>, so the explicit overrides are removed.

Changes

• src/command.ts — drop --base-url from GLOBAL_OPTIONS
• src/registry.ts — drop --base-url line from help text
• src/types/flags.ts — drop baseUrl? field from GlobalFlags
• src/config/loader.ts — shorten baseUrl chain to file.base_url || file.oauth?.resource_url || REGIONS[region] || REGIONS.global
• src/args.ts — explicitly reject --base-url with a CLIError(USAGE) pointing at mmx config set --key base_url, so users no longer see their input silently dropped
• src/errors/handler.ts — refresh stale MINIMAX_BASE_URL example in the fetch failed branch comment

Tests

• test/config/loader.test.ts — 7 new cases for the baseUrl source chain (file wins, region fallback, env ignored, malformed URL fallback). Uses MMX_CONFIG_DIR for cross-platform isolation.
• test/args.test.ts — 3 new cases for the --base-url rejection (space form, = form, and a happy-path smoke test).

bun test: 358/358 non-SDK tests pass. The 14 SDK failures are pre-existing and unrelated (live API integration tests that need network + a valid key).
bun run lint: clean.
bun run typecheck: clean.

Side effects

• mmx search query --base-url https://... now errors with Flag --base-url was removed. and a migration hint.
• export MINIMAX_BASE_URL=... is silently ignored — the user's ~/.mmx/config.json (or region default) wins. No warning by design.
• To set a custom base URL: mmx config set --key base_url --value <url>.

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