feat: add Deep Code (dc) as a supported agent

[pernielsentikaer]

Summary

• Add Deep Code (deepcode) to the dmux agent registry.
• Detect deepcode from PATH and common install locations (npm global,
  Homebrew, local bin).
• Launch initial prompts via tmux send-keys since Deep Code is an
  interactive TUI agent with no CLI-level prompt or permission flags.
• Add Deep Code pane attention heuristics coverage, registry tests,
  official agent icon assets, and docs references.

Deep Code CLI facts

• Binary: deepcode
• Install: npm install -g @vegamo/deepcode-cli
• GitHub: https://github.com/lessweb/deepcode-cli
• Docs: https://api-docs.deepseek.com/quickstart/agentintegrations/deepcode
• Config directory: ~/.deepcode/settings.json
• Model: DeepSeek-V4 Pro / Flash
• Resume: interactive /resume or /continue slash commands within the TUI
  (no CLI --continue flag)
• Permission bypass: interactive slash commands only (no CLI bypass flag)

Prompt transport choice

This uses promptTransport: 'send-keys' with sendKeysSubmit: ['Enter'],
sendKeysPostPasteDelayMs: 200, and sendKeysReadyDelayMs: 2000.

Deep Code is an interactive terminal application (like Grok Build and Cline
CLI)‚ boots into a TUI, reads input from the terminal, and does not
accept prompts via CLI arguments, --prompt, or stdin. The send-keys
transport pastes the initial prompt into the running TUI after the agent
has finished booting, matching the pattern already used for grok, cline,
and crush.

Deep Code has no CLI-level permission flags. permissionFlags is left empty
since all permission decisions happen through interactive slash commands
within the TUI. No resumeCommandTemplate is set because Deep Code uses the
interactive /resume and /continue commands rather than a CLI flag. Pane
resume falls back to buildAgentCommand, which relaunches deepcode.

Deep Code is registered as defaultEnabled: false, consistent with the
optional-agent pattern for newly supported agents.

Test plan

pnpm install
pnpm exec vitest --run __tests__/agentLaunch.test.ts
pnpm exec tsc --noEmit
pnpm --filter dmux-docs run build