fix(startup): unify agent startup with beacon + instructions in CLI prompt (#977)

All agents now receive their startup beacon + role-specific instructions via
the CLI prompt, making sessions identifiable in /resume picker while removing
unreliable post-startup nudges.

Changes:
- Rename FormatStartupNudge → FormatStartupBeacon, StartupNudgeConfig → BeaconConfig
- Remove StartupNudge() function (no longer needed)
- Remove PropulsionNudge() and PropulsionNudgeForRole() functions
- Update deacon, witness, refinery, polecat managers to include beacon in CLI prompt
- Update boot to inline beacon (can't import session due to import cycle)
- Update daemon/lifecycle.go to include beacon via BuildCommandWithPrompt
- Update cmd/deacon.go to include beacon in startup command
- Remove redundant StartupNudge and PropulsionNudge calls from all startup paths

The beacon is now part of the CLI prompt which is queued before Claude starts,
making it more reliable than post-startup nudges which had timing issues.
SessionStart hook runs gt prime automatically, so PropulsionNudge was redundant.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/session/startup.go

@@ -4,12 +4,11 @@ package session
 import (
 	"fmt"
 	"time"
-
-	"github.com/steveyegge/gastown/internal/tmux"
 )
 
-// StartupNudgeConfig configures a startup nudge message.
-type StartupNudgeConfig struct {
+// BeaconConfig configures a startup beacon message.
+// The beacon is injected into the CLI prompt to identify sessions in /resume picker.
+type BeaconConfig struct {
 	// Recipient is the address of the agent being nudged.
 	// Examples: "gastown/crew/gus", "deacon", "gastown/witness"
 	Recipient string
@@ -27,27 +26,17 @@ type StartupNudgeConfig struct {
 	MolID string
 }
 
-// StartupNudge sends a formatted startup message to a Claude Code session.
-// The message becomes the session title in Claude Code's /resume picker,
-// enabling workers to find predecessor sessions.
+// FormatStartupBeacon builds the formatted startup beacon message.
+// The beacon is injected into the CLI prompt, making sessions identifiable
+// in Claude Code's /resume picker for predecessor discovery.
 //
 // Format: [GAS TOWN] <recipient> <- <sender> • <timestamp> • <topic[:mol-id]>
 //
 // Examples:
 //   - [GAS TOWN] gastown/crew/gus <- deacon • 2025-12-30T15:42 • assigned:gt-abc12
-//   - [GAS TOWN] deacon <- mayor • 2025-12-30T08:00 • cold-start
-//   - [GAS TOWN] gastown/witness <- self • 2025-12-30T14:00 • handoff
-//
-// The message content doesn't trigger GUPP - CLAUDE.md and hooks handle that.
-// The metadata makes sessions identifiable in /resume.
-func StartupNudge(t *tmux.Tmux, session string, cfg StartupNudgeConfig) error {
-	message := FormatStartupNudge(cfg)
-	return t.NudgeSession(session, message)
-}
-
-// FormatStartupNudge builds the formatted startup nudge message.
-// Separated from StartupNudge for testing and reuse.
-func FormatStartupNudge(cfg StartupNudgeConfig) string {
+//   - [GAS TOWN] deacon <- daemon • 2025-12-30T08:00 • patrol
+//   - [GAS TOWN] gastown/witness <- deacon • 2025-12-30T14:00 • patrol
+func FormatStartupBeacon(cfg BeaconConfig) string {
 	// Use local time in compact format
 	timestamp := time.Now().Format("2006-01-02T15:04")
 
@@ -91,3 +80,18 @@ func FormatStartupNudge(cfg StartupNudgeConfig) string {
 
 	return beacon
 }
+
+// BuildStartupPrompt creates the CLI prompt for agent startup.
+//
+// GUPP (Gas Town Universal Propulsion Principle) implementation:
+//   - Beacon identifies session for /resume predecessor discovery
+//   - Instructions tell agent to start working immediately
+//   - SessionStart hook runs `gt prime` which injects full context including
+//     "AUTONOMOUS WORK MODE" instructions when work is hooked
+//
+// This replaces the old two-step StartupNudge + PropulsionNudge pattern.
+// The beacon is processed in Claude's first turn along with gt prime context,
+// so no separate propulsion nudge is needed.
+func BuildStartupPrompt(cfg BeaconConfig, instructions string) string {
+	return FormatStartupBeacon(cfg) + "\n\n" + instructions
+}