Define config schema and parser

[tnez]

Context
V1 uses static TOML config loaded from $XDG_CONFIG_HOME/.hud/config.toml. See docs/v1-scope.md.

Acceptance criteria

• Define the smallest useful TOML shape for one home dashboard, command-backed panels, and fire-and-forget actions.
• Parse config once at the boundary into typed Rust values.
• Missing config produces a clear user-facing error.
• Invalid config produces a clear user-facing error with enough context to fix it.
• Unit tests cover valid config, missing required fields, and invalid shapes.

Out of scope

• Multiple config formats.
• Runtime config editing.
• Persistent state.

[tnez]

Implementation is complete in PR #11: #11. I could not close this issue from here because merging PR #11 into main was blocked pending explicit approval for the default-branch mutation. The PR is green and links this issue for automatic closure on merge.