docs(mcp): add setup and authenticated-fetch usage guidance

[mherod]

Problem

Users need a tested setup path and an accurate description of what the MCP server exposes. The draft adds command help, but README/VitePress currently contain no MCP guide or authenticated-fetch examples.

Location: README.md:138, docs/guide/cli-usage.md:6, docs/guide/security.md:1, docs/.vitepress/config.mjs:34, src/cli/cli.ts:249, src/mcp/server.ts:48.
Type: documentation / feature delivery.
Priority: Medium. Estimate: S (1–2 hours; no matching size/s label exists).
Owner: @mherod. Readiness: blocked for final validation. Depends on #593, #594 and #595; drafting may start earlier, but examples must be verified against the finished command. No blocked label exists in the repository.

Found in: 2026-09-07 MCP implementation draft on top of bb7bd67841a0d72f750c0468fd08ca479e39e629.

Latest handoff snapshot: the implementation is preserved in local commit 96b5531 on codex/add-mcp-server, and the working tree is clean. These file/line references describe that implementation; they are not files guaranteed at the earlier base commit. Continue from this branch/commit through a PR to main, because direct pushes to main are prohibited. Type-check passes; MCP build, test, runtime and live authenticated-request verification remain outstanding in this planning handoff. A local commit does not establish remote delivery or CI success.

Proposed documentation

Add a VitePress MCP guide, link it from README/navigation, and update CLI/security pages. Cover the three tools and a client configuration using:

{
  "mcpServers": {
    "get-cookie": {
      "command": "get-cookie",
      "args": ["mcp", "--allow-origin", "https://app.example.com"]
    }
  }
}

This is a proposed example until tested against the built implementation. Provide an explicit local-build command/path variant for development; do not imply that the existing published npm version already contains MCP.

Research Findings

Official MCP documentation explains command/argument configuration for local servers and using Inspector to exercise them. Use documentation appropriate to the selected SDK/client generation.

• Connect local MCP servers
• MCP SDK client transport

Recommended Approach

1. Document startup-origin configuration, profile discovery and a metadata-only cookie query followed by an authenticated GET example.
2. Explain both gates for returning raw cookie values and the separate startup switch for methods beyond GET/HEAD. State that GET itself can still have server-side effects.
3. Describe manual redirects, header/body limits, deadlines, response encoding/truncation, no Set-Cookie persistence and how HTTP failures differ from tool failures.
4. Explain data handling: raw cookies are credentials, response bodies can contain private account data, and remote response text is untrusted. MCP output may reach client transcripts/model context.
5. Preserve actual browser limitations and provide actionable troubleshooting for origin denial, missing profiles, locked/unreadable stores and unsupported encryption. Keep setup advice consistent with the selected client and OS.

Acceptance Criteria

• README, CLI guide and VitePress navigation link to the MCP guide.
• Client setup and all three tool examples are executed against the built CLI with synthetic fixtures or recorded manual evidence.
• Documentation accurately covers origin/method/value gates, profile/container semantics, limits, redirects, response encoding and errors.
• The guide clearly distinguishes the current local build from any published version; no release is claimed before it exists.
• Safari named profiles, partitioned-cookie exclusions, Windows encryption limitations and non-browser-fetch semantics are stated without claiming browser-equivalent session behaviour.
• Cookie values are absent from examples/logs; private-response and client-transcript implications are explained.
• pnpm run check-links and pnpm run docs pass; no new formatting errors.

Duplicate Check and Existing Constraints

No existing MCP documentation issue was found. Read full bodies and comments of #560 (open, 1 comment; latest 2026-06-27), #557 (open, 0), and #572 (open, 0). Their existing scope already captures Safari named-profile storage research, ABE limitations and Windows DPAPI diagnostics respectively; this documentation issue references those limitations rather than duplicating their implementation work. No freshness comments were added.