docs(readme): lead with status and quick start, make sandbox docs config-first

[locez]

What changed

Both entry documents buried what a new reader needs first: the README spent its
first ~250 lines on sandbox and permission internals before saying what Merry is
and how to run it, and SANDBOX described the sandbox layers before telling
operators how to configure anything.

README.md now orders sections by reader priority:

positioning + two design promises -> Status And Focus -> What Works
-> Quick Start -> Configure -> Use The CLI -> Sandbox And Permissions
-> Multi-Tool Execution -> Embed Merry -> Verify -> Documentation

The sandbox section keeps what a user actually configures ([cli] sandbox,
approval_policy, [permissions] network and path settings, host integrations),
then summarizes the boundary in ### How The Boundary Works and points to
SANDBOX.md.

SANDBOX.md (new tracked file) takes the sandbox and permission detail that
used to live in the README, ordered Setup -> Configure -> How it works:

• Setup - requirements, the built-in host probe, the Ubuntu 24.04+ bubblewrap
  AppArmor profile, and kernel restrictions.
• Configure - sandbox mode, approval policy, permissions, host integrations.
• How it works - sandbox layers, mount plan and scanning bounds, enforcement,
  host integration reachability, SSH agent, GPG agent, capability retention, and
  permission review without a terminal.

No fact was dropped; the removed README material was moved rather than deleted.

Why

Status, focus, and the Quick Start path are what a visitor needs first; sandbox
internals are reference material that belongs behind a link. Configuration
instructions also have to precede the explanation of the mechanism.

Verification

• Link and anchor checker over both files: 18 links, all resolving, including
  cross-file fragments such as README.md#sandbox-and-permissions and
  #ubuntu-2404-and-newer-the-bubblewrap-apparmor-profile.
• Code fences balanced in both files (36 and 16 markers).
• git diff --check clean; only the two intended files are modified.
• Line-level comparison of the new files against HEAD confirmed every removed
  line is a rewording, a heading-level change, or a relocated sentence.
• Docs-only change: neither file is read by tests or build scripts, so the Rust
  and Python suites were not run.

Notes

main integration in this repository is fast-forward-only; this branch is a
single commit on top of main.