[FEATURE]: Document snapshot config option

[altendky]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Note: PR #4753 attempted this but was closed without merge. This issue tracks the same gap.

Describe the enhancement you want to request

The snapshot config field (added in 28a4517) is undocumented. It lacks both a section in the config docs page (packages/web/src/content/docs/config.mdx) and a .describe() annotation on its schema definition.

Users frequently discover the hard way that snapshots can consume enormous disk space in large repos, with no obvious way to disable them. Multiple issues reference this:

• HUGE snapshot folder #6845 — HUGE snapshot folder
• Very slow session due to 98GB of git snapshots #3182 — Very slow session due to 98GB of git snapshots
• Opencode using my 20 GB storage #10532 — Opencode using my 20 GB storage
• Snapshot feature consumes excessive disk space (170GB+ in my case) #17753 — Snapshot feature consumes excessive disk space (170GB+)
• new terminal using opencode in home dir with snapshot: true will occupy huge storage #16336 — new terminal using opencode in home dir with snapshot: true will occupy huge storage
• [FEATURE]: Snapshot lifespan ought be configurable. #10626 — Snapshot lifespan ought be configurable

What should be documented:

1. A .describe() annotation on the snapshot schema field in config.ts
2. A "Snapshot" section in config.mdx explaining:
   • What snapshots do (track file changes, enable undo/revert)
   • How to disable them ("snapshot": false)
   • The tradeoff: disabling means session undo/revert becomes unavailable
   • When you might want to disable: large repos, many submodules, disk space concerns

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• docs: document snapshot config option #17862: Nearly identical request to document the snapshot config option — same description, same referenced issues, same suggested changes. This issue was recently closed.