docs: add File-Write Safety feature guide (upstream epic #1375)

[easonLiangWorldedtech]

Docs: File-Write Safety feature guide (upstream epic #1375)

Adds the user-facing documentation for the file-write safety series: how a write is guarded, when it is rejected and what the remediation means, per-write checkpoints, per-step change cards, rollback, and the related settings. Mirrors the DTE-style docs PR: one self-contained page, wired into the sidebar, with screenshots.

New pages

• File-Write Safety (docs/features/file-write-safety.md): how a write is guarded (observed vs unobserved), the four rejection messages and their meaning, per-write checkpoints, per-step change cards (summary/full), rollback (file and step), the checkpoints settings, the default write path, and where the data lives.

Wiring

• sidebars.ts: features/file-write-safety added after features/checkpoints.
• docs/features/index.md: File-Write Safety listed under Workflow Management.
• docs/basic-usage/the-chat-interface.md: new Change Cards section (what a card shows, per-file and per-step rollback).
• docs/features/checkpoints.mdx: per-write checkpoint bullet added to the configuration list.
• Tool reference pages (apply-patch.md, edit-file.md, edit.md, write-to-file.md): each gains a short Write safety section describing the guarded publish the tool goes through.

Images

Four screenshots under /img/file-write-safety/ (exact component crops, light theme):

• change-card-summary.png — summary change card
• change-card-full.png — full (detailed) change card
• rollback-state.png — change card in the rolled-back state
• settings-checkpoints.png — checkpoints settings section

Verification

• pnpm build passes (Docusaurus production build, no new warnings beyond the pre-existing update-notice banners).
• All internal links resolve (sidebar entry, feature index, cross-references from the chat-interface and checkpoints pages).
• No version numbers or build artifacts referenced in the prose.

Related code series: upstream epic Zoo-Code-Org/Zoo-Code#1375 (trial aggregate Zoo-Code-Org/Zoo-Code#1413).