feat(roles): add config-based role definition system (Phase 1)

Replace role beads with embedded TOML config files for role definitions.
This is Phase 1 of gt-y1uvb - adds the config infrastructure without
yet switching the daemon to use it.

New files:
- internal/config/roles.go: RoleDefinition types, LoadRoleDefinition()
  with layered override resolution (builtin → town → rig)
- internal/config/roles/*.toml: 7 embedded role definitions
- internal/config/roles_test.go: unit tests

New command:
- gt role def <role>: displays effective role configuration

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

internal/config/roles/dog.toml

@@ -0,0 +1,23 @@
+# Dog role definition
+# Town-level workers for cross-rig tasks. Dispatched by Deacon.
+
+role = "dog"
+scope = "town"
+nudge = "Check your hook for work assignments."
+prompt_template = "dog.md.tmpl"
+
+[session]
+pattern = "gt-dog-{name}"
+work_dir = "{town}/deacon/dogs/{name}"
+needs_pre_sync = false
+start_command = "exec claude --dangerously-skip-permissions"
+
+[env]
+GT_ROLE = "dog"
+GT_SCOPE = "town"
+
+[health]
+ping_timeout = "30s"
+consecutive_failures = 3
+kill_cooldown = "5m"
+stuck_threshold = "2h"