docs(exploration): desktop filesystem as a governed capability (0270)

[crs48]

Summary

Exploration 0270 — a thought experiment on giving the desktop (Electron) app access to the user's local file system, both to load context into the workspace ("digital brain") and to let plugins/agents do real file work (compose documents, synthesize analysis).

• Builds on unimplemented 0127 (ingestion/indexing design) and adds the actor half: filesystem as a sixth ModuleCapabilities entry, brokered per-plugin like secrets/network, enforced by a new guardFs facade beside guardStore/guardedFetch.
• Grants are device-local roots picked via native dialog; renderer and plugin code never see paths (opaque IDs only). Marketplace-tier plugins get no direct fs — they consume indexed nodes via guardStore.
• Writes: managed workbench folder → save-as powerbox dialogs → in-place edits deferred.
• Agents: xnet_fs_* MCP tools on :31416 with plan→approve→apply, plus a lethal-trifecta taint gate (sessions that read local files require approval for outbound actions/shares).
• Platform matrix: Electron full; Chromium web "lite" adapter possible later; Safari/Firefox/mobile none.

Docs-only — no changeset needed.

🤖 Generated with Claude Code

[github-actions]

This PR needs a changelog entry.

Add a fragment file to the PR (it lands with the merge — nothing is
written to main separately, and the PR number is filled in at deploy):

node scripts/changelog/new.mjs --title "Short, benefit-first headline" \
  --summary "One sentence on what the user can now do." \
  --tags app,ai --highlight "A specific user-visible point"

No user-facing change? Add the skip-changelog label instead.

[github-actions]

Preview: https://xnet.fyi/pr/389/app/