feat(install): add --remove, --validate, multi-agent uninstall

[ajianaz]

What

Add --remove (uninstall) and --validate (post-install config validation) flags to cora install.

Why

Issue #430 requires multi-agent config validation and complete install/uninstall lifecycle. Previously cora install could only add entries — there was no way to remove Cora from agent configs, and no verification that written configs still parse correctly.

How

cora install --remove

• Removes the mcpServers.cora entry from all detected agents
• Works for both JSON/JSONC and YAML formats
• Gracefully skips agents that don't have a cora entry
• Combines with --dry-run to preview removal

cora install --validate

• After install/remove, re-reads each modified config file
• Validates JSON/JSONC parses without error (round-trip check)
• Reports per-file status with green check / red cross
• YAML validation deferred (format-specific parser needed)

Changes

• src/commands/install.rs: +uninstall_json_agent(), +validate_json_config(), refactored install_agent() to dispatch install vs remove, refactored execute_install() for remove mode + validation block
• src/main.rs: added --remove and --validate CLI flags
• src/mcp/tools.rs: updated InstallOptions construction

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, no issues found

Related Issues

Closes #430

Checklist

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