Support generic environment config overrides

[ewgdg]

Summary

Add generic environment-variable overrides for cliamp configuration values, mainly so secrets do not need to be stored in ~/.config/cliamp/config.toml.

Proposal

Support environment variables with this shape:

CLIAMP_CONFIG__<KEY>
CLIAMP_CONFIG__<SECTION>__<KEY>

Mapping examples:

CLIAMP_CONFIG__PROVIDER=ytmusic
CLIAMP_CONFIG__YTMUSIC__CLIENT_ID=...
CLIAMP_CONFIG__YTMUSIC__CLIENT_SECRET=...
CLIAMP_CONFIG__NAVIDROME__PASSWORD=...
CLIAMP_CONFIG__PLEX__TOKEN=...
CLIAMP_CONFIG__JELLYFIN__TOKEN=...

These map to TOML values like:

provider = "ytmusic"

[ytmusic]
client_id = "..."
client_secret = "..."

[navidrome]
password = "..."

Nested sections can use additional __ separators, for example:

CLIAMP_CONFIG__PLUGINS__LASTFM__API_KEY

maps to:

[plugins.lastfm]
api_key = "..."

Precedence

defaults < config.toml < environment overrides < CLI flags

Scope

• Support config values generically, including top-level keys and existing config sections.
• Do not execute shell commands from config or environment variables.
• No stdin config support needed.
• Keep provider-specific environment variable sprawl out of the codebase.

Why

• Keeps secrets out of ~/.config/cliamp/config.toml.
• Works with secret managers and deployment tools that inject environment variables.
• Small, safe, cross-platform feature.
• Avoids adding separate env var names for every provider and secret.