Add internal/model package for shared data types

[castrojo]

Description

Extract a pure data-types package (internal/model) to serve as the contract between all packages. This prevents wizard from becoming a god package and eliminates import cycle risk.

Rationale

Without this, ignition imports wizard for the config struct, install imports wizard, and tui imports wizard. Having a leaf package with zero dependencies that everyone imports cleanly breaks this coupling.

Implementation

// internal/model/model.go
package model

// InstallConfig is the accumulated wizard state passed to ignition generation.
type InstallConfig struct {
    Hostname    string
    Disk        DiskSelection
    Network     NetworkConfig
    Users       []UserConfig
    Sysexts     []Sysext
    ConfigMode  ConfigMode
    ExternalURL string // only when ConfigMode == ExternalURL
}

type ConfigMode int
const (
    ConfigModeGuided ConfigMode = iota
    ConfigModeExternal
)

type DiskSelection struct {
    ByIDPath   string // /dev/disk/by-id/...
    Model      string
    Serial     string
    SizeBytes  uint64
    Transport  string // sata, nvme, usb
    Removable  bool
}

type NetworkMode int
const (
    NetworkModeDHCP NetworkMode = iota
    NetworkModeStatic
)

type NetworkConfig struct {
    Mode      NetworkMode
    Interface string
    Address   string // CIDR notation
    Gateway   string
    DNS       []string
    Hostname  string
}

type UserConfig struct {
    Username     string
    SSHKeys      []string
    PasswordHash string // optional, bcrypt/SHA-512
}

type Sysext struct {
    Name    string
    URL     string
    Version string
    Channel string
}

Dependency Graph

model ← (leaf, zero imports — everyone can depend on it)
runner ← probe, install
validate ← tui, ignition
probe ← wizard/tui
bakery ← wizard/tui
ignition ← install, wizard
install ← wizard
wizard ← tui, cmd/knuckle

Acceptance Criteria

• internal/model/model.go exists with all types above
• Zero imports (only stdlib if needed for enums)
• All other packages reference model.InstallConfig instead of defining their own structs
• go build ./... passes

[castrojo]

Implemented in commits 8f7422e..174b76f. All packages built, tested, and verified in QEMU VM.

[castrojo]

Reopening — audit found model types diverge from issue spec.

Criteria Status

• internal/model/model.go exists with types
• Zero imports (only stdlib)
• All other packages reference model.InstallConfig — verified: wizard, ignition, install, tui, probe all import model
• go build ./... passes

What's missing from the type spec

The issue body defines specific types. Comparing spec vs actual:

1. ConfigMode enum — NOT IMPLEMENTED. The spec defines ConfigModeGuided/ConfigModeExternal. The code uses IgnitionURL string as an implicit mode flag instead. No explicit enum.
2. NetworkConfig.Hostname — Missing from actual struct. Hostname is on InstallConfig directly.
3. Sysext.Channel — Missing. Actual SysextEntry has no Channel field.
4. Struct naming — Issue spec says DiskSelection, actual is DiskInfo. Issue says Sysext, actual is SysextEntry. Minor but noted.

These are relatively minor compared to other issues, but the types don't match what was specified.

[castrojo]

Completed — model package exists with full types, butane compilation integrated in ignition package