feat(client,mcp): signatures vertical

[dougborg]

Summary

Build a client.signatures helper + MCP tools covering Front's
signature management surface (7 endpoints).

Status

Deferred — filed as part of the #16 workspace-admin partition.
Front handles signature selection at send time based on inbox / channel
configuration; agents composing replies via the existing drafts /
messages tools don't need to enumerate or set signatures explicitly.
Pick up if a concrete "agent picks signature per message" or "agent
authors a signature" use case appears.

Scope

• Generated modules under frontapp_public_api_client/api/signatures/:
  • Reads: list_team_signatures / list_teammate_signatures /
    get_signatures (no company-level list)
  • Mutations: create_team_signature / create_teammate_signature /
    update_signature / delete_signature
• New helpers/signatures.py + client.signatures lazy property.
• MCP tools — reads cached, mutations gated with confirm_or_preview.
• Tests + docs.

Path quirk

• No company-level list endpoint — to enumerate every signature in the
  workspace, the helper has to walk teams + teammates first.

Umbrella

Part of #16 (workspace admin partition). Tier 3.