feat: add Jcode prompt overlay integration

[mountainsea-lab]

Adds prompt-level RTK integration for Jcode via .jcode/prompt-overlay.md and global $JCODE_HOME/prompt-overlay.md support.

Changes

• Adds jcode as an rtk init --agent target.
• Adds project-scoped install support: rtk init --agent jcode.
• Adds global install support: rtk init -g --agent jcode.
• Writes .jcode/prompt-overlay.md or $JCODE_HOME/prompt-overlay.md, falling back to ~/.jcode/prompt-overlay.md.
• Adds uninstall support that removes only the RTK marked block.
• Adds Jcode-specific instructions under hooks/jcode/.
• Updates supported agents documentation.
• Adds tests for create, idempotency, uninstall, env resolution, and CLI dispatch.

Notes

This is intentionally prompt-level only. It does not transparently rewrite commands because current Jcode builds do not expose a command-mutating before-tool hook.

Testing

• rtk cargo check passed locally on Windows.
• rtk cargo fmt -- --check passed locally on Windows.
• rtk cargo test previously passed locally: 1909 passed, 6 ignored.