agents: add more MCP clients (Cherry Studio, Witsy, 5ire, BoltAI)

[fschrhunt]

Umbrella for broadening client coverage. Each of these is an MCP-capable client that isn't in src/cli/agents/built-in.ts yet:

• Cherry Studio
• Witsy
• 5ire
• BoltAI

One PR per client (keeps review small), each following the Adding a new agent walkthrough in CONTRIBUTING.md: registry entry in built-in.ts, a test/registry.test.ts case, CHANGELOG entry. Confirm each client's actual MCP config path/key as part of its PR.

Pick one, comment which you're taking, and link your PR back here. New contributors very welcome — see the per-client good first issues for Continue / Goose / Jan / LM Studio / Warp for the same shape.

[fschrhunt]

Worked through this umbrella. Three of the four are clean json-merge adds and are up as separate PRs (one per client, as requested); each confirms the client's actual config path/key/shape against upstream source before wiring it:

• BoltAI → agents: add BoltAI client #46 — local servers at ~/.boltai/mcp.json, canonical mcpServers. Standard entry.
• Witsy → agents: add Witsy client #47 — Claude-compatible top-level mcpServers map in settings.json (Electron userData dir), unioned with its native array at read time. Standard entry.
• 5ire → agents: add 5ire client #48 — mcpServers in mcp.json (Electron userData dir), but needs isActive: true to auto-connect, so it adds a small 5ire entry shape (same pattern as Jan's active).

Cherry Studio — can't be done with json-merge (recommend parking)

Cherry Studio is not a clean add and I've left it off. It does not store MCP servers in any plain JSON object-map file:

• Shipping v1.x: servers live in a redux-persist store backed by the renderer's localStorage — i.e. a Chromium LevelDB blob under <userData>/Local Storage/leveldb/. The slice is mcp.servers, an array of rich typed records, not a name-keyed mcpServers map.
• v2 (main, unreleased): servers move to a SQLite DB at <userData>/cherrystudio.sqlite (table mcp_server, one row per server).
• The mcpServers JSON you see in Settings is an in-app editor that (de)serializes Redux state — there's no on-disk file for us to merge into.

The only sanctioned programmatic install path is the deep link cherrystudio://mcp/install?servers=<base64(json)>, which requires the app running plus a user trust confirmation, and writes a richer typed record than {command, args}.

So Cherry Studio is the same class of work as Continue (#25) and Goose (#26): it needs a new install mechanism (a cherrystudio:// deep-link launcher), not a registry entry. Suggest splitting it into its own gated issue and parking pending demand, rather than holding this umbrella open for it.