feat(cli): rename fern mcp install to fern agent install

[devin-ai-integration]

Description

Frees the fern mcp * namespace for the customer's own generated MCP server (Mathieu's ask), by moving the Fern-agent MCP installer to fern agent install.

fern mcp install shipped in CLI 5.91.0, so it stays wired to the same handler, hidden from help (describe: false, same treatment as write-overrides), and logs The 'mcp install' command is deprecated. Use 'fern agent install' instead. before running.

Changes Made

• addAgentCommand registers fern agent install; addMcpCommand keeps fern mcp install as a hidden deprecated alias. Options and handler are shared via addInstallAgentOptions / runInstallAgent, so the two paths cannot drift.
• PostHog events distinguish the two: fern agent install vs the existing fern mcp install.
• src/commands/mcp/ → src/commands/agent/ (pure move), and the two user-facing hints inside installMcpServer.ts now say fern agent install.
• CLI changelog entry (feat).

Testing

• Unit tests added/updated — existing mcpConfig tests moved with the directory; pnpm turbo run test --filter @fern-api/cli → 638 passed.
• Manual testing completed — pnpm fern-dev:build, run from outside a Fern project:

$ fern-dev --help | grep agent
  fern-dev agent                        Manage the Fern agent

$ fern-dev agent install --help
Connect your coding agent to the Fern agent's MCP server using your Fern login
Options: --client [claude|cursor|codex]  --organization, --org

$ fern-dev mcp install --client claude --org fern
The 'mcp install' command is deprecated. Use 'fern agent install' instead.
The MCP server authenticates a user, so a FERN_TOKEN cannot be used. ...

fern mcp and fern mcp install no longer appear in --help.

Link to Devin session: https://app.devin.ai/sessions/50a5d4aae6764bf78406b2752ab933f3
Open in Devin Desktop: https://app.devin.ai/desktop/session/50a5d4aae6764bf78406b2752ab933f3?variant=devin

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[devin-ai-integration]

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[nitpickybot]

AI Review Summary

Straightforward rename with a hidden deprecated alias; logic is shared so the two paths can't drift. Only minor nits: the deprecated fern mcp group's demandCommand message points at the new namespace (confusing if someone types bare fern mcp), and runInstallAgent's argv type is hand-rolled rather than inferred from addInstallAgentOptions.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.