feat(cli): ship the terminal client for macOS in every release

[mahype]

Why

The terminal client (torromail) built on macOS but could not store a secret there: the platform store fell through to secret-tool, which a Mac does not have. Releases also shipped it only for Linux and Windows.

What

• torromail-keychain (new, macOS only): creates keychain items with the same access list the app's teamScopedAccess() uses — any binary signed by the TorroMail team reads them without a dialog. It is the one crate allowed unsafe, confined to its sys module; the workspace still forbids it everywhere else. An unsigned build never replaces an item it cannot read, because only it could read the replacement.
• torromail-control: KeychainStore is the macOS platform store.
• torromail-tui: keychain dialogs are off for the whole run (as in the server). The background check installs a launchd agent on macOS (com.torromail.check, every 15 minutes). Notifications go through Notification Center.
• Release: scripts/build-cli-macos.sh builds torromail + torromail-mcp universal, signs them with the Developer ID under the hardened runtime, has them notarized and packs torromail-<version>-universal-macos.tar.gz. The existing macOS release job runs it, and the publish step now requires a download for every system (macOS, Linux x86_64 and aarch64, Windows).
• scripts/install-macos.sh installs a signed local build. README, RELEASING.md and AGENTS.md are updated.

Verification

• cargo test --workspace --locked: all 41 suites pass. There is a new launchd test for macOS.
• Real keychain, signed with the team's development identity:
  • A second binary reads what the first one wrote without a dialog, also under the hardened runtime.
  • Items the app created (mail passwords, client-key-*) are readable.
  • An unsigned binary is refused without a dialog.
• torromail check with the installed build logged into all 6 real accounts via the app's keychain items.
• scripts/build-cli-macos.sh ran end to end locally with NOTARIZE=0: universal binaries, Team ID in the signature, smoke test, tarball.
• Not verified yet: notarization inside GitHub Actions. That only runs on a tag, so a -rc tag after merge is the safe first run.

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