fix(config): don't export empty GT_ROOT/BEADS_DIR in AgentEnv (#385)

* fix(config): don't export empty GT_ROOT/BEADS_DIR in AgentEnv

Fix polecats not having GT_ROOT environment variable set. The symptom was
polecat sessions showing GT_ROOT="" instead of the expected town root.

Root cause: AgentEnvSimple doesn't set TownRoot, but AgentEnv was always
setting env["GT_ROOT"] = cfg.TownRoot even when empty. This empty value
in export commands would override the tmux session environment.

Changes:
- Only set GT_ROOT and BEADS_DIR in env map if non-empty
- Refactor daemon.go to use AgentEnv with full AgentEnvConfig instead
  of AgentEnvSimple + manual additions
- Update test to verify keys are absent rather than empty

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

* fix(lint): silence unparam for unused executeExternalActions args

The external action params (beadID, severity, description) are reserved
for future email/SMS/slack implementations but currently unused.

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

---------

Co-authored-by: julianknutsen <julianknutsen@users.noreply.github>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: max <steve.yegge@gmail.com>

internal/config/env.go

@@ -81,8 +81,14 @@ func AgentEnv(cfg AgentEnvConfig) map[string]string {
 		env["GIT_AUTHOR_NAME"] = cfg.AgentName
 	}
 
-	env["GT_ROOT"] = cfg.TownRoot
-	env["BEADS_DIR"] = cfg.BeadsDir
+	// Only set GT_ROOT and BEADS_DIR if provided
+	// Empty values would override tmux session environment
+	if cfg.TownRoot != "" {
+		env["GT_ROOT"] = cfg.TownRoot
+	}
+	if cfg.BeadsDir != "" {
+		env["BEADS_DIR"] = cfg.BeadsDir
+	}
 
 	// Set BEADS_AGENT_NAME for polecat/crew (uses same format as BD_ACTOR)
 	if cfg.Role == "polecat" || cfg.Role == "crew" {