#7 Add inbox CLI command for discovering shared grants via appview

Implemented opake inbox command — closes the Phase 2 inbox gap.

opake-core changes:

• Appview error variant in error.rs (distinct from Xrpc)
• client/appview_auth.rs: sign_appview_request() pure function (Ed25519, no clock dep)
• client/appview_types.rs: InboxGrant + InboxResponse (Deserialize, camelCase)
• client/appview.rs: fetch_inbox() + fetch_inbox_all() with auto-pagination via Transport trait
• client/mod.rs: wired 3 new modules with pub use re-exports

opake-cli changes:

• config.rs: appview_url: Option<String> field + resolve_appview_url() (flag > env > config > error)
• commands/inbox.rs: InboxCommand with --long and --appview flags, matches shared.rs format pattern
• commands/mod.rs + main.rs: wired Inbox variant into dispatch

All 323 tests pass. Clippy clean on opake-core + opake-cli.