fix: guard unsaved prompt-stage edits when switching stages (#6021)

[atomantic]

Summary

Editing a prompt stage's template, model tier, or timeout override and then clicking another stage in the Stages list silently discarded the edits: the click called setSelectedStage directly, and the selectedStage effect immediately re-fetched and overwrote the editor. Prompt templates run to hundreds of lines, so a mis-tap in the 120+ row stage list destroyed real work with no warning and no undo.

The Stages tab now uses the same unsaved-edit guard the Job Skills tab has had since #3939:

• savedStageTemplate / savedStageConfig hold the last server-confirmed values (set when getPrompt resolves and after a successful save), so an edit undone back to the original stops counting as dirty.
• A dirty editor shows an "Unsaved changes" note under the stage heading and an "Unsaved" badge on its list row.
• Clicking a different stage parks it in pendingStage and turns that row into an InlineConfirmRow ("Discard" / "Keep editing") instead of switching — the URL, the fetch, and the typed text all hold. No window.confirm, per client/src/AGENTS.md.
• Confirming discards and switches; cancelling, undoing the edit, or saving clears the prompt. Re-clicking the already-open stage backs out of the question.
• Saving re-baselines only while the same stage is still open, so a PUT that lands after the user moved on can't stamp the outgoing stage's text onto the incoming one.

Deep linking is untouched — selection still lives entirely in the ?stage= param.

Test plan

• client/src/pages/PromptManager.test.jsx gains a PromptManager stage unsaved-edit guard suite (10 cases): dirty on template edit, dirty on config-only change, revert-to-clean, arming the confirm row without switching, "Keep editing", "Discard", no prompt after a save, disarm on undo, back-out on re-click, and no prompt from a clean editor. 9 of the 10 fail against main (the 10th is the clean-switch regression guard).
• cd client && npm test — 834 files / 10230 tests pass.
• cd client && npx vite build — clean.

Closes #6021

https://claude.ai/code/session_01VjkWVTfzKyRuAv3HEsspwN