Add README scaffolding to create wizard: default-on toggle, seeded template, authored-content preservation, and atomic write transaction - #457

Preserve pre-existing temporary files

Use a unique transaction-owned temporary path (and clean it up) instead of ${mutation.path}.tmp. When facet create --force targets a project that already has, for example, README.md.tmp or facet.json.tmp, this write overwrites that unrelated authored file and the subsequent rename removes it; the transaction never captures or restores temporary-path preimages, so the promised rollback cannot recover the data.

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Return scaffold failures as a result value

Expose the transaction failure as a discriminated writeScaffold result instead of throwing it. Filesystem failures such as an unwritable target or a conflicting path are expected create outcomes that the command can render and exit from normally; this throw instead bypasses both the headless and wizard command paths and reaches the process-level catch as an “unexpected” exit (code 2).

AGENTS.md reference: AGENTS.md:L262-L266

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Dead applied tracking array

applied is pushed to on every successful mutation but is never read. rollbackApplied always receives preimages (the full pre-transaction preimage set), not applied, so the tracking does nothing. Restoring every preimage when only a subset was applied is still correct — restoring a preimage for an untouched path is a no-op (rmSync with { force: true } on a path that was never written). The array and its push call can be removed entirely to avoid misleading future readers into thinking it drives rollback selection.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

Avoid overwriting an existing README without --force

When the target directory has no facet.json but does have a pre-existing README.md (for example, creating a facet in an existing repository root), overwrite protection permits the operation because it checks only facet.json in create/index.ts:85. This new default mutation then silently replaces the authored README even though the user did not pass --force; guard planned output paths or require explicit overwrite consent before writing it.

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Remove transaction-created directories during rollback

If a later mutation fails after this call has created parent directories—for example, a pre-existing commands file makes the command write fail after a skill was written—the rollback removes the planned files but never removes newly created skills/<name> directories. The target is therefore not restored to its pre-transaction state despite the transaction contract; record which directories were newly created and prune them after restoring preimages.

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Honor --no-readme in the wizard

When users run facet create --no-readme without another headless content flag, decideCreate deliberately selects the wizard, but the flag is never passed into its initial form state. The wizard therefore starts with README enabled and will create and declare README.md unless the user manually disables it, despite the advertised --no-readme opt-out. Apply this flag to the wizard state (or make it select an appropriate noninteractive path) as well as the headless options.

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Run probe mutations only once

For each test whose steps callback changes form state, including all of the new mutation tests, ctx gets a new identity after the update, so this effect runs again and invokes the setters again. Because each setter returns a new form object even when the values are unchanged, this produces an unbounded render/effect loop rather than a settled result. Depend only on the stable steps callback (or otherwise guard the setup effect) so the probe performs its sequence once.

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