feat(agents): custom CLI agent profiles (#1473)

[nwparker]

Closes #1473.

What

Adds user-defined CLI agent profiles — named variants of a built-in TuiAgent with their own launch command and optional env vars. The motivating ask in #1473 was "two Claudes pointed at different provider URLs"; this lands a general mechanism for that and similar workflows (custom Codex with separate OPENAI_API_BASE, Claude with --dangerously-skip-permissions baked in, etc.).

Where it surfaces

• Settings → Agents — new Custom Agents section with add/edit/delete, base-agent picker, command field, and env key/value pairs. Each profile gets a "Set as default" affordance.
• Default Agent picker in Settings → Agents and in the workspace composer — custom profiles render as additional pills/items with the base agent's icon plus a wrench overlay.
• New Workspace composer (full + quick) — the agent combobox lists custom profiles inline; selecting one routes the launch through the profile's command + env.
• Tab-bar Quick Launch — custom profiles whose baseAgent is detected on PATH appear in the launch dropdown.

How it's wired

• New CustomAgentProfile = { id, label, baseAgent: TuiAgent, command, env? } in src/shared/types.ts.
• GlobalSettings.customAgents: CustomAgentProfile[] (default []).
• GlobalSettings.defaultTuiAgent widens to accept { kind: 'custom', id }. Stale custom defaults fall back to auto, matching how uninstalled built-ins fall back.
• buildAgentStartupPlan / buildAgentDraftLaunchPlan accept an optional customProfile; when present, the launch base command comes from the profile (with env rendered as a POSIX shell prefix KEY='value' …) instead of cmdOverrides[agent] ?? config.launchCmd. The base agent still drives prompt-injection mode, telemetry kind, and paste-draft logic, so custom agents ride the existing flow with zero downstream changes.
• Env shell prefix piggybacks on the existing startup-command path (local-pty-shell-ready.ts writes startup commands to the user's interactive shell) — no PTY-spawn changes needed.
• New helpers: src/shared/custom-agent-profile.ts (env quoting + base command), src/renderer/src/lib/custom-agent-resolve.ts (lookup + default-pref resolver).

Selection model

Composer state stays minimal: tuiAgent: TuiAgent + customAgentId: string | null (parallel slots). When the user selects a custom profile, the picker sets tuiAgent to its baseAgent and records the profile id, so existing call sites that only know about TuiAgent (telemetry, paste-draft, trust preflight) continue to work unchanged.

Tests

• src/shared/custom-agent-profile.test.ts (new) — POSIX/Windows env quoting, single-quote escaping, command resolution.
• src/renderer/src/lib/tui-agent-startup.test.ts — added cases for both buildAgentStartupPlan and buildAgentDraftLaunchPlan with a custom profile (incl. that profile command + env override the per-agent cmdOverrides).

npm run typecheck clean. npm run lint clean (0 errors; 7 pre-existing warnings untouched). Vitest passes for everything except the pre-existing better-sqlite3 native-module mismatch in orchestration/runtime tests (failing identically on master).

Notes / out of scope

• Env shell-prefix uses POSIX quoting; on Windows the settings hint advises wrapping with cmd /c … if env vars matter. A first-class Windows path can come later.
• Migration: customAgents defaults to []; existing users keep working unchanged until they explicitly pick a custom profile.

Made with Orca 🐋