feat: add config package with types and JSON serialization

Types:
- TownConfig: main town configuration (config/town.json)
- RigsConfig: rigs registry with BeadsConfig per rig
- AgentState: agent state with role, session, extras

Features:
- Load/Save functions for all types
- Version compatibility checks
- Required field validation
- Creates parent directories on save

Closes gt-f9x.1

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/types.go

@@ -0,0 +1,45 @@
+// Package config provides configuration types and serialization for Gas Town.
+package config
+
+import "time"
+
+// TownConfig represents the main town configuration (config/town.json).
+type TownConfig struct {
+	Type      string    `json:"type"`       // "town"
+	Version   int       `json:"version"`    // schema version
+	Name      string    `json:"name"`       // town identifier
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// RigsConfig represents the rigs registry (config/rigs.json).
+type RigsConfig struct {
+	Version int                 `json:"version"`
+	Rigs    map[string]RigEntry `json:"rigs"`
+}
+
+// RigEntry represents a single rig in the registry.
+type RigEntry struct {
+	GitURL      string       `json:"git_url"`
+	AddedAt     time.Time    `json:"added_at"`
+	BeadsConfig *BeadsConfig `json:"beads,omitempty"`
+}
+
+// BeadsConfig represents beads configuration for a rig.
+type BeadsConfig struct {
+	Repo   string `json:"repo"`   // "local" | path | git-url
+	Prefix string `json:"prefix"` // issue prefix
+}
+
+// AgentState represents an agent's current state (*/state.json).
+type AgentState struct {
+	Role       string         `json:"role"`              // "mayor", "witness", etc.
+	LastActive time.Time      `json:"last_active"`
+	Session    string         `json:"session,omitempty"`
+	Extra      map[string]any `json:"extra,omitempty"`
+}
+
+// CurrentTownVersion is the current schema version for TownConfig.
+const CurrentTownVersion = 1
+
+// CurrentRigsVersion is the current schema version for RigsConfig.
+const CurrentRigsVersion = 1