Add grut config get command to read a single config value

[jongio]

What

Adds grut config get <key>, a scripting-friendly way to read one resolved config value by dotted key.

• Scalars print as a bare value so they pipe cleanly: grut config get git.default_branch prints main.
• Sections print as TOML: grut config get preview prints the whole preview table.
• Unknown keys print Error: unknown config key: <key> and exit 1 (usage is silenced so scripts get a clean one-line error).

Defaults are applied, so a key the user never set still returns the effective value. The lookup round-trips the resolved config through TOML into a generic map, so key names track the existing TOML tags with no second source of truth.

Deadlock fix

Every grut config subcommand (including the existing config check) deadlocked at runtime before this change. main.go wrapped slog's built-in default handler in the crash-tail handler and re-set it as the default. slog.SetDefault rewires the stdlib log package back through the new default, so the old bridge handler formed a cycle that hung the first time anything logged at INFO or above. config.Load triggers that through icon-mode detection. The crash tail now feeds from a concrete discard handler (matching how the TUI sets up logging), so the ring buffer still captures entries for crash reports without writing to the console.

Tests

• Unit tests for scalar, nested scalar, section, unknown top-level key, unknown nested key, descending into a scalar, load error, and slice output.
• go build ./..., go vet ./..., mage lint (0 issues), and mage deadcode (OK) all pass.

Closes #341