feat(pm-desktop): validate required tools on launch

[danii1]

Summary

DAN-6: PM Desktop is not a self-contained agent runtime — ticket work needs Git and at least one supported agent CLI on PATH. Until now that only failed later, as a spawn error after the user had already opened a project.

On launch the app now probes the same PATH it uses to spawn agents (including common GUI-launch locations that a dock/launcher start would otherwise miss). If a required tool is missing, a blocking screen names it and how to install it. Check again (or focusing the window after installing) re-runs the probe. When the check passes there is no extra confirmation step — the usual welcome / last-project restore continues.

• Required: Git + at least one registered agent harness CLI
• Optional extras (sandbox CLIs, etc.) are not required and do not block
• IPC: pm:validate-required-tools
• Docs: desktop README + PM quick-start prerequisites

Test plan

• lefthook pre-push (bun run --filter '*' test and typecheck)
• bun run --filter @devintern/pm-desktop test (covers validate-tools, tool-validation, RequiredToolsGate)
• bun run dev in packages/pm-desktop with Git + an agent CLI present — no gate, last project restores as before
• Hide Git or every agent CLI from PATH (or launch from a dock-like env that omits ~/.local/bin / Homebrew) — gate lists the missing tool and install hint
• Install the missing tool, return to the app or click Check again — gate clears and the project UI appears
• Probe failure path: gate shows “Couldn't check required tools” and still offers Check again

[danii1]

@devintern-internal bump the version of the pm-desktop for release