#11 Add config command with get and set subcommands

New CLI command: opake config get <key> and opake config set <key> <value>. Reads/writes config.toml programmatically. Keys: pds_url, trust_mode, appview_url, etc. opake config get with no key dumps all config. Validates values on set (e.g. trust_mode only accepts 'standard'/'zero-trust', URLs must parse). Does NOT expose secrets (session tokens, etc.).

config get/set should also handle default_did — replace the existing set-default command with opake config set default_did <did>. The dedicated set-default command gets removed in favor of the unified config interface. Full key list: pds_url, default_did, trust_mode, appview_url.

Config struct Default impl should populate appview_url from OPAKE_APPVIEW_URL env var if set. This lets deployment environments (e.g. Docker, systemd) inject the AppView URL without touching config.toml. Same pattern as OPAKE_DATA_DIR. Env var takes precedence over config file value — check env on load, not just on Default.