feat(config): publish JSON schema for editor autocomplete

[stephenleo]

Summary

• Adds a cship config-schema subcommand (via schemars) that emits a JSON Schema for the [cship] config section.
• Publishes the generated schema as docs/public/config-schema.json, served at https://cship.dev/config-schema.json once the docs site redeploys — usable via "$schema" = 'https://cship.dev/config-schema.json' in cship.toml/starship.toml for editor autocomplete (mirrors Starship's config-schema.json).
• CI now diffs a freshly generated schema against the committed one so it can't silently drift from src/config.rs.
• Docs: new "Editor Schema" section in docs/configuration.md.

Closes #196

Test plan

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test (591 passed)
• cargo build --release
• cargo run -- config-schema | diff - docs/public/config-schema.json (matches, same check CI runs)

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