Remove marker-based editing, use packages.json state file (#32)

## Summary

This PR removes the marker-based flake.nix editing system in favor of a
JSON state file (`packages.json`) that tracks installed packages. The
flake.nix is now completely regenerated on every change instead of being
edited in place.

- Add new `state` module with `PackageState` struct for managing
packages.json
- Add `migration` module for one-time automatic migration from
marker-based format
- Simplify template generation (no more markers in output)
- Update install/uninstall/list commands to use state file
- Remove `editor.rs` (marker editing no longer needed)
- Remove `--force` flag from install (no longer relevant)
- Remove ~1200 lines of marker manipulation code

**State file format**
(`~/.config/nixy/profiles/<profile>/packages.json`):
```json
{
  "version": 1,
  "packages": ["ripgrep", "fzf", "bat"],
  "custom_packages": [
    {
      "name": "neovim",
      "input_name": "neovim-nightly",
      "input_url": "github:nix-community/neovim-nightly-overlay",
      "package_output": "packages"
    }
  ]
}
```

**Benefits:**
- Simpler code (removed complex string manipulation)
- Cleaner flake.nix (no marker comments)
- Easier debugging (state visible in JSON)
- Better testability (state separate from generation)
- One-time automatic migration for existing users

Fixes #26

## Test plan

- [x] Run `cargo test` - all 138 tests pass (75 unit + 63 integration)
- [x] Run `cargo clippy` - no warnings
- [ ] Test migration: create a flake with markers, run nixy command,
verify packages.json is created
- [ ] Test install: `nixy install hello` - verify packages.json updated
and flake.nix regenerated
- [ ] Test uninstall: `nixy uninstall hello` - verify package removed
from state and flake
- [ ] Test list: `nixy list` - verify packages listed from state file

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: yusukeshib

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "nixy-rs"
-version = "0.1.6"
+version = "0.1.7"
 edition = "2021"
 rust-version = "1.80"
 description = "Homebrew-style wrapper for Nix using flake.nix"
@@ -17,6 +17,8 @@ dirs = "5"
 thiserror = "2"
 colored = "2"
 regex = "1"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
 self_update = { version = "0.42", default-features = false, features = ["rustls"] }
 
 [dev-dependencies]

src/cli.rs

@@ -66,10 +66,6 @@ pub struct InstallArgs {
     /// Install from local nix file
     #[arg(long, short)]
     pub file: Option<PathBuf>,
-
-    /// Force regeneration even if flake.nix has custom modifications
-    #[arg(long)]
-    pub force: bool,
 }
 
 #[derive(Args)]