feat(core): extract agent_config module for config R/W

[ajianaz]

What

Extract config read/write/merge logic from install.rs into a standalone, testable agent_config module.

Why

The config handling logic (JSONC comment stripping, BOM handling, trailing comma tolerance, mcpServers injection) was buried inside install.rs with no unit tests. Issue #432 asks for a proper module so that:

1. Future features (cora uninstall, cora config check) can reuse the same R/W logic
2. Edge cases (BOM, JSONC comments, trailing commas) are handled correctly
3. String values containing // or ,} are not corrupted during read-modify-write

How

• New module src/commands/agent_config.rs with public API:
  • read_json_config() / write_json_config() — JSON/JSONC read+write
  • read_yaml_config() / write_yaml_config() — YAML read+write
  • json_has_cora() / json_add_cora() / json_remove_cora() — mcpServers CRUD
  • yaml_has_cora() / yaml_add_cora() / yaml_remove_cora() — YAML equivalents
  • read_config() / write_config() — format-agnostic dispatch
  • strip_bom() — UTF-8/16 BOM stripping
  • strip_jsonc_comments() — string-aware state machine (not regex)
  • strip_trailing_commas() — string-aware state machine (not regex)
• Refactored install.rs to delegate to agent_config, removing ~120 lines of duplicated logic
• 21 unit tests covering: comment stripping, BOM, trailing commas, string corruption edge cases, add/remove/has for both JSON and YAML

String-aware comment stripping

Previous regex approach (//.*) would corrupt URLs in config values like "https://example.com". The new state machine tracks string context to avoid this.

Testing

• cargo test --features tree-sitter — 893 pass, 0 fail
• cargo clippy --all-targets --features tree-sitter -- -D warnings — 0 warnings
• cargo fmt --all -- --check — clean
• Cora review — passed (fixed string-corruption finding before commit)

Related Issues

Closes #432

Checklist

• Tests pass
• No clippy warnings
• Formatted
• PR description follows template
• Commit messages follow conventional commits
• Linked to relevant issues