#75 Add account management commands and --as flag

Implements #52 + #53 from the plan:

Part 1 (#52): Account management commands

• config.rs: resolve_handle_or_did(), remove_account() with tests
• commands/accounts.rs: list all accounts, mark default with *
• commands/set_default.rs: switch default by handle or DID
• commands/logout.rs: remove account config + data directory
• main.rs: global --as flag, new command variants

Part 2 (#53): Execute trait refactor

• Execute::execute now takes &CommandContext
• login.rs standalone execute() (no trait)
• run_with_context() generic helper in main.rs
• Removed load_client_default(), load_identity_default()
• resolve_context delegates to resolve_handle_or_did

174 tests passing, clippy clean.