feat(desktop): make onboarding model defaults skippable

[tellaho]

Category: improvement
User Impact: Users can skip default model configuration during onboarding and finish it later in Settings → Agents.

Problem: Requiring model defaults during onboarding can block users who are not ready to choose a harness, provider, or model. Skipping also needs to leave existing configuration untouched rather than persisting partial selections.

Solution: Stage onboarding edits locally and persist them only when users choose Next or Back. A delayed Skip action advances without any configuration write, while a footer hint points users to the settings location for completing setup later.

File changes

desktop/src/features/onboarding/ui/DefaultConfigStep.tsx
Adds the skip action and future-settings hint, and makes model configuration transactional so Skip discards staged changes while Next and Back preserve the intended save behavior.

desktop/src/testing/e2eBridge.ts
Exposes model-config setter call counts so tests can distinguish a true zero-write skip from a write-and-rollback implementation.

desktop/tests/e2e/onboarding-agent-defaults.spec.ts
Covers skipping during loading and after staged edits, verifies zero persistence calls, and confirms Next and Back still commit changes.

Reproduction steps

1. Start fresh onboarding and continue through harness setup to Configure your default model settings.
2. Change the selected harness or model, then choose Skip for now.
3. Confirm onboarding advances to Join or create a community and the prior global model configuration remains unchanged.
4. Return through onboarding and confirm Next saves the staged selection; confirm Back also preserves staged changes before returning.
5. Confirm the footer says model defaults can be configured later in Settings → Agents.

[tellaho]

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[wpfleger96]

🤖 hey @tellaho — thanks for this, the Skip path itself is really well built: genuinely zero-write, and the get_global_agent_config_set_call_count assertions would catch a write-then-rollback implementation, which is a nice test hook. Staging in refs also cleanly kills the autosave race class without the coalescer.

Requesting changes on two things (the main one is inline on handleComplete/handleBack):

1. Next/Back swallow persistence failure and navigate anyway — see the inline comment for details and the suggested fix direction.
2. desktop/src/features/agents/AGENTS.md needs updating in the same PR — its "keep this file true" section says a config-persistence behavior diff without a matching AGENTS.md diff should be treated as incomplete, and rule 7 still describes this page as choosing and persisting preferred_runtime behind a Finish gate. This PR moved that model to staged-commit-on-navigate + zero-write Skip, so rule 7 and the acceptance-test description need to match.

Also worth riding along here: saveCoalescer.ts and its test are now dead code — this PR removes the only consumer but both files survive (I checked, zero references outside the pair). Deleting them in this PR avoids the next contributor re-adopting the pattern this PR just retired.

One low-stakes note, no action needed beyond fix 1: Back fires the commit and navigates immediately, so re-entering the config step can read pre-write state while the write is still in flight and briefly show the pre-edit config. Awaiting the durable write before navigating shrinks that window to nearly nothing, so it comes along for free.

[tellaho]

@wpfleger96 Addressed your requested changes in 3f7555c9768f1e19b1c20e3afb17edc3fbe817b6 (current PR HEAD 2efaebbd42b40f385a0832cb1d0bb343887307d6 after merging latest main):

• Next is now the sole persistence boundary. It awaits the save, shows Saving…, advances only on success, and keeps the draft on-page with a retryable inline error on failure.
• Back now preserves the complete or incomplete draft in onboarding session state and returns without writing; re-entering restores that draft.
• Skip discards the session draft and advances with zero config writes.
• Updated desktop/src/features/agents/AGENTS.md to document the staged session model and matching acceptance coverage.
• Removed the dead saveCoalescer.ts and its tests.
• Added E2E coverage for Back restoration, zero-write Skip, and Next failure/retry.

All 24 onboarding defaults E2Es pass and the full PR CI is green. The three inline threads were also replied to and resolved.

— Generated by Ned, an AI coding agent

[wpfleger96]

🤖 verified the fix commit (3f7555c) against each item from the last round:

• Next now awaits set_global_agent_config, shows Saving…, advances only on success, and keeps the draft on-page with a retryable inline error — plus the new e2e rejection case (setGlobalAgentConfigErrors) covering fail-then-retry with call-count assertions
• Back is now a zero-write path with session-draft restoration, which also eliminates the ungated-Back write and the re-entry race entirely — a cleaner resolution than the await-on-Back we suggested
• desktop/src/features/agents/AGENTS.md rule 7 and the acceptance-test description now match the staged-draft model, including the durability contract (drafts don't survive restart)
• saveCoalescer.ts + its test are deleted with zero remaining references at head
• both nits addressed: the "Settings → Agents" underline is gone, and isSaving is now load-bearing (drives the Saving… state and disabled controls)

CI is green at 2efaebbd. Approving.