docs(settings): align guide with current behavior

[8thEdition]

Reason

docs/guide/settings.md contained seven statements that no longer matched the current server/frontend behavior, including document history retention, document caps, task-manual write-back, lesson defaults, theme capabilities, and two adjustable settings. Readers could follow the guide and either do the wrong thing or miss available controls.

Builder level: L1 / R0 — documentation-only correction with no product logic, schema, data, or deployment impact.

Changes

1. Document history (docs/guide/settings.md): distinguish the normal three-version retention from the ten-version lifecycle-document retention and clarify which documents have reset/default behavior.
2. Caps and task manuals (docs/guide/settings.md): describe the shared two-way cap floor, lifecycle-document caps, agent-created manuals, and the separate learning write-back operation.
3. Settings and themes (docs/guide/settings.md): document agent-token and warden-credential controls and the full theme editing surface.

Verification

Outcome: Success

• Re-read the seven affected statements against origin/main at 5319aecc4422f82d15f5b550f50d496f10c0f16c using git show on the referenced source files.
• Confirmed the working tree contains only docs/guide/settings.md.
• git diff --check passed.
• No product test suite was run because this is a documentation-only change.

Checks

• Single-file documentation change
• No generated files or API specifications changed
• Mainline source references recorded in task T-147