feat(p10): archivage automatique des sessions terminées (rétention configurable)

- DB: migration #10 (sessions.archived_at + index), helpers archive/unarchive/archiveExpired (soft-archive, jamais de DELETE)
- core/retention-settings.ts: session_retention_days (défaut 30, 0=jamais, 1-3650) + session_purge_days (off)
- core/session-archive.ts: scheduler start/stop/sweep (calqué DiscoveryService), câblé dans runDaemon
- pty-manager.list({includeArchived}) + SessionSummary.archived + emitHistoricalUpdate
- routes: GET /sessions?includeArchived, POST/DELETE /sessions/:id/archive, POST /sessions/archive-now
- protocole additif: message WS session_archived (relayé par la gateway, abonnés 'sessions')
- settings: retentionDays/purgeDays exposés et validés dans PATCH /settings
- web: store sessions (showArchived + archive/unarchive), SessionsListView (toggle/badge/actions), SettingsView (slider rétention), i18n EN+FR
- tests: retention-settings + session-archive (vitest) + settings-routes étendu ; acceptance-p10.mjs (sweep, event WS, resume d'une session archivée → 201, rétention=0)

packages/web/src/i18n/en.ts

@@ -47,6 +47,11 @@ export default {
     hideDiscovered: 'Hide external history',
     showHidden: 'Show hidden',
     hideHidden: 'Hide hidden',
+    archive: 'Archive',
+    unarchive: 'Unarchive',
+    archivedBadge: 'archived',
+    showArchived: 'Show archived',
+    hideArchived: 'Hide archived',
     clients: 'no client | 1 client | {n} clients',
     exitCode: 'exit code {code}',
     statusLabel: {
@@ -342,6 +347,8 @@ export default {
     sessionHidden: 'Session hidden',
     sessionUnhidden: 'Session shown',
     sessionsHidden: 'no session hidden | 1 session hidden | {n} sessions hidden',
+    sessionArchived: 'Session archived',
+    sessionUnarchived: 'Session unarchived',
     repoAdded: 'Repository added',
     repoRemoved: 'Repository removed',
     worktreeCreated: 'Worktree created',
@@ -413,6 +420,11 @@ export default {
     removeRoot: 'Remove',
     scanInterval: 'Re-scan interval (minutes)',
     scanIntervalHint: '0 disables periodic scanning. Changes apply on the next daemon restart.',
+    // Sessions (rétention / archivage auto)
+    sessions: 'Sessions',
+    sessionsHint: 'Finished sessions older than the retention window are automatically archived (hidden from the list by default). Nothing is deleted — archived sessions stay resumable and forkable.',
+    retentionDays: 'Auto-archive after (days)',
+    retentionDaysHint: '0 never archives. Archived sessions are hidden by default; toggle "Show archived" in the sessions list to see them.',
     // Claude CLI
     claudeCli: 'Claude CLI',
     claudeCliHint: 'Where Arboretum finds the `claude` binary and its data. Handy when the daemon runs as a service with a minimal PATH that does not include ~/.local/bin.',