docs: recover missing CLI docs + types for recently-merged security PRs

[proggeramlug]

Three PRs that landed via admin-bypass today reference docs files
that don't actually exist in the tree, plus three perry/system PRs
that didn't update the hand-maintained `types/` file. This is the
docs follow-up.

What's missing

PR	Issue	Surface
#981	#505	`docs/src/cli/sandbox-buildrs.md` linked from SUMMARY but never created
#988	#504	`docs/src/cli/emit-attest.md` linked from SUMMARY but never created
#969	#501	No docs at all — no `capabilities.md`, no SUMMARY entry
#976	n/a	`getOSVersion()` added to runtime + manifest, not to `types/perry/system/index.d.ts`
#972	#917	`shareText` / `shareUrl` added to runtime + manifest, not to `types/`
#974	#675	`appGroupSet` / `appGroupGet` / `appGroupDelete` added to runtime + manifest, not to `types/`

`docs/src/cli/lockdown.md` (#496) and `docs/src/cli/emit-sandbox.md`
(#506) did land in their respective PRs and are unaffected.

What this PR does

• Creates the three missing `docs/src/cli/*.md` files with content
  derived from each PR's description (workflow, opt-in mechanisms,
  capability tokens, related-PR cross-references).
• Adds the SUMMARY.md entry for `capabilities.md`.
• Adds the six new `perry/system` signatures to
  `types/perry/system/index.d.ts` with the same docstring style as
  the surrounding entries.

Why not in the original PRs

Most of these PRs were rebased + admin-merged locally during today's
loop. The docs gaps were pre-existing in the PRs themselves (the
SUMMARY entries pointed at files that were never committed); the
`types/` gaps are the standard "manifest entry was added but the
hand-maintained types file wasn't touched" pattern.

The auto-generated `docs/api/perry.d.ts` + `docs/src/api/reference.md`
already cover the new symbols and are up to date on main.

Test plan

• `mdbook build docs/` succeeds with no dead links.
• Spot-check each new doc renders sanely; cross-references resolve.
• Spot-check `types/perry/system/index.d.ts` against the runtime
  dispatch table in `crates/perry-dispatch/src/lib.rs` — signatures match.

Pure docs-only diff. No code changes.