feat(onboarding): add one-time feature tips

[johannesjo]

Summary

This is the first step of an optional onboarding. Rather than a tour of the app up front, which on a fresh install would point at empty panels, it shows one-time tips. Each tip appears the first time its feature has something to work on:

• Change tour: appears by the tour button once the active task has changes and its agent has stopped working. It says which model receives the diff, since the tip stands in for the button's hover help while it shows.
• Readiness: appears when the Finish dialog opens. It explains the evidence and checks. If the project has no checks yet, it also offers a "Configure checks" button, the same condition the evidence panel uses.
• Delegating to sub-tasks: appears in the New Task panel once other tasks exist and the selected agent supports orchestration. It offers an example prompt.

How tips behave

• One tip at a time. After a tip is dismissed or its feature used, the next waits a minute. A tip waits 600ms before it shows, so it doesn't pop up while a dialog opens or for a task the user is only switching past.
• Tips float over the page with a caret pointing at their control, so they never move content.
• Inside a dialog, the tip sits beside the panel, or else under or over it. It overlaps the panel only when there is no room around it, so it doesn't cover the dialog's buttons.
• A tip hides while its anchor is scrolled out of view or covered by another dialog. Dialog panels now carry data-dialog-id, so a tip can tell which dialog is on top without relying on DOM order.
• A tip follows its anchor every frame while the anchor moves, then checks every 250ms. Using the feature counts as having seen the tip. Escape dismisses a tip without closing the dialog around it, and clicking its buttons leaves focus where the user was typing.

Settings and saved state

• featureHintsEnabled and seenFeatureHints are saved in state.json.
• New installs start with tips on. Existing installs, whose state has no featureHintsEnabled, start with them off, so long-time users aren't suddenly shown tips.
• A new "Show feature tips" setting turns tips on or off. Turning it on shows every tip again.
• New installs also no longer see the keyboard shortcut migration notice, since there is nothing to migrate.

Code

• src/store/onboarding.ts: the single tip slot, the cooldown and the seen list.
• src/lib/feature-hint-placement.ts: pure placement logic, with unit tests.
• src/components/FeatureHint.tsx: the tip card.

Test plan

• npm run check and npm run check:static (typecheck, lint, knip, dependency-cruiser, Prettier)
• npm run test:changed; the push hook also ran npm test
• New tests:
  • store: the single slot, the cooldown after dismissing or using a feature, turning tips on again
  • saving and loading: new install versus existing install, unknown tip IDs dropped
  • placement rules
  • the tip component: settle delay, clipped anchors, dialog stacking, slot hand-off, follow loop and its cleanup, Escape, focus kept on click
  • Finish dialog wiring: the readiness tip appears, and no setup offer shows once checks exist
• All three tips rendered with the real component and stylesheet in a Vite and Playwright harness, in dark and light themes, with wide and narrow dialogs.
• Not run in the Electron app itself: there is no Electron binary in this environment. Worth a look at the three tips in npm run dev.

Not in this PR

• Screen readers aren't told when a tip appears. Tips mounted on the page body are hard to reach by keyboard, so this needs a reachability design first.
• The floating-card styling is repeated in several popovers across the app. A shared helper would touch components outside this feature.

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https://claude.ai/code/session_017Cf9ACGvzjah4kib977pkZ