feat(lifecycle): validate shared symlinks at daemon startup (A2)

Wire symlinks.ValidateAll into the lifecycle manager so the daemon
refuses to start if any configured account is missing one of the
shared-state symlinks or if a link diverges from the canonical target.

Previously, a missing link on a freshly deployed VM would silently
create a divergent state tree per account (duplicate JSONL transcripts,
broken undo history) — exactly the failure mode the symlinks package
(A1) was introduced to prevent.

The check runs once at startup before EnsureAllSessions, guarding a
single well-defined invariant: "every account home shares the same
projects/, file-history/ and session-env/ roots". No auto-heal on
divergence — we fail fast with an explicit error so the operator fixes
it manually rather than one account's state being overwritten.

Part of Phase 1 Chantier A — Failover robuste.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/lifecycle/manager.go

@@ -4,11 +4,13 @@ package lifecycle
 
 import (
 	"context"
+	"fmt"
 	"log"
 	"time"
 
 	"forge.secuaas.ovh/olivier/claude-failover/internal/config"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/state"
+	"forge.secuaas.ovh/olivier/claude-failover/internal/symlinks"
 	"forge.secuaas.ovh/olivier/claude-failover/internal/tmux"
 )
 
@@ -47,6 +49,35 @@ func (m *Manager) Run(ctx context.Context) {
 	}
 }
 
+// ValidateSharedSymlinks verifies that every configured account home has
+// the three shared-state symlinks (session-env, file-history, projects)
+// in place and pointing at the canonical shared targets.
+//
+// Called once at daemon startup BEFORE sessions are recreated. A missing
+// or divergent link would silently fork the state tree between the two
+// accounts, breaking failover. We fail fast so the operator fixes it
+// before any work is in flight.
+//
+// EnsureForAccount creates missing links but refuses to touch divergent
+// ones — see internal/symlinks for the rationale.
+func (m *Manager) ValidateSharedSymlinks() error {
+	if len(m.config.Accounts) == 0 {
+		return fmt.Errorf("[lifecycle] no accounts configured — cannot validate shared symlinks")
+	}
+	homes := make([]string, 0, len(m.config.Accounts))
+	for _, acc := range m.config.Accounts {
+		if acc.Home == "" {
+			return fmt.Errorf("[lifecycle] account %q has empty home — refusing to continue", acc.Name)
+		}
+		homes = append(homes, acc.Home)
+	}
+	if err := symlinks.ValidateAll(homes, symlinks.RequiredShared); err != nil {
+		return fmt.Errorf("shared symlinks invalid, refusing to start: %w", err)
+	}
+	m.logger.Printf("[lifecycle] shared symlinks OK for %d account(s)", len(homes))
+	return nil
+}
+
 // EnsureAllSessions creates all configured sessions that are not yet present in tmux.
 // It is intended to be called once at daemon startup before Run is launched.
 func (m *Manager) EnsureAllSessions() {