refactor(types): remove Gas Town type constants from beads core (bd-w2zz4)

Remove Gas Town-specific type constants (TypeMolecule, TypeGate, TypeConvoy,
TypeMergeRequest, TypeSlot, TypeAgent, TypeRole, TypeRig, TypeEvent, TypeMessage)
from internal/types/types.go.

Beads now only has core work types built-in:
- bug, feature, task, epic, chore

All Gas Town types are now purely custom types with no special handling in beads.
Use string literals like "gate" or "molecule" when needed, and configure
types.custom in config.yaml for validation.

Changes:
- Remove Gas Town type constants from types.go
- Remove mr/mol aliases from Normalize()
- Update bd types command to only show core types
- Replace all constant usages with string literals throughout codebase
- Update tests to use string literals

This decouples beads from Gas Town, making it a generic issue tracker.

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

cmd/bd/export_obsidian.go

@@ -32,20 +32,14 @@ var obsidianPriority = []string{
 	"⏬", // 4 = backlog/lowest
 }
 
-// obsidianTypeTag maps bd issue type to Obsidian tag
-// Note: Gas Town-specific types (agent, role, rig, convoy, slot) are now labels.
-// The labels will be converted to tags automatically via the label->tag logic.
+// obsidianTypeTag maps bd issue type to Obsidian tag (core types only)
+// Gas Town types are custom types and will use their issue_type value as a tag.
 var obsidianTypeTag = map[types.IssueType]string{
-	types.TypeBug:          "#Bug",
-	types.TypeFeature:      "#Feature",
-	types.TypeTask:         "#Task",
-	types.TypeEpic:         "#Epic",
-	types.TypeChore:        "#Chore",
-	types.TypeMessage:      "#Message",
-	types.TypeMergeRequest: "#MergeRequest",
-	types.TypeMolecule:     "#Molecule",
-	types.TypeGate:         "#Gate",
-	types.TypeEvent:        "#Event",
+	types.TypeBug:     "#Bug",
+	types.TypeFeature: "#Feature",
+	types.TypeTask:    "#Task",
+	types.TypeEpic:    "#Epic",
+	types.TypeChore:   "#Chore",
 }
 
 // formatObsidianTask converts a single issue to Obsidian Tasks format