feat(doctor): add fn doctor CLI + 14 functions for system management

Adds `fn doctor` read-only diagnostic command with subcommands artefacts,
services, sync, uses-functions, unused, and --json flag for agents.
Each subcommand wraps a registry function in functions/infra/.

New functions:
- artefact_doctor, services_status, pc_locations_drift,
  audit_uses_functions, find_unused_functions (Go diagnostics)
- backup_sqlite_db, rotate_backups, wait_for_http, wait_for_port,
  port_kill, tail_journal, pre_commit_hook_install (bash utilities)
- notify_telegram (Go HTTP)
- backup_all pipeline (tag launcher)

Plus prior session leftovers (scan_secrets_in_dirty, append_diary_entry,
git utilities, http_session_cookie_middleware, compile/full-git pipelines).

Fixes pc_locations_drift filepath.Join bug with absolute dir_path.
Documents fn doctor in CLAUDE.md, .claude/rules/fn_doctor.md (rule 23),
docs/architecture.md, CHANGELOG.md (2026-05-07), and diary entry.

First fn doctor uses-functions run found drift in 7/12 apps (deuda
para sincronizar app.md con imports reales).

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

functions/infra/pc_locations_drift.md

@@ -0,0 +1,61 @@
+---
+name: pc_locations_drift
+kind: function
+lang: go
+domain: infra
+version: "1.0.0"
+purity: impure
+signature: "func PcLocationsDrift(registryRoot string, pcID string) ([]LocationDrift, error)"
+description: "Compara la tabla pc_locations contra el estado real del disco para el PC actual. Detecta tres tipos de drift: carpetas activas que no existen, entradas missing cuya carpeta reaparece, y artefactos indexados en disco sin fila en pc_locations."
+tags: [doctor, sync, pc_locations, drift]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports:
+  - "bufio"
+  - "database/sql"
+  - "fmt"
+  - "os"
+  - "path/filepath"
+  - "strings"
+  - "github.com/mattn/go-sqlite3"
+tested: true
+tests:
+  - "active entry whose folder is missing reports missing_on_disk"
+  - "active entry with existing folder reports no drift"
+  - "missing entry whose folder reappears reports status_should_be_active"
+  - "indexed app on disk without pc_locations row reports untracked_on_disk"
+test_file_path: "functions/infra/pc_locations_drift_test.go"
+file_path: "functions/infra/pc_locations_drift.go"
+params:
+  - name: registryRoot
+    desc: "Ruta absoluta a la raiz del registry (donde vive registry.db)."
+  - name: pcID
+    desc: "Identificador del PC (ej: 'home-wsl'). Si vacio, se lee de la primera linea no vacia de ~/.fn_pc."
+output: "Slice de LocationDrift con todos los discrepancias encontradas. Vacio si no hay drift. Error solo si no se puede abrir registry.db."
+---
+
+## Ejemplo
+
+```go
+drifts, err := PcLocationsDrift("/home/lucas/fn_registry", "")
+if err != nil {
+    log.Fatal(err)
+}
+for _, d := range drifts {
+    fmt.Printf("[%s] %s/%s -> %s\n", d.Issue, d.EntityType, d.EntityID, d.DirPath)
+}
+```
+
+## Notas
+
+Read-only — nunca modifica la BD ni el disco. Util como paso inicial de `fn doctor sync`.
+
+La funcion `readFnPC` es compartida en el paquete infra (lee `~/.fn_pc` ignorando lineas vacias y comentarios con `#`).
+
+Tipos de issue reportados:
+- `missing_on_disk`: entrada `active` cuya carpeta no existe → sugerir cambiar status a `missing`
+- `status_should_be_active`: entrada `missing` cuya carpeta existe → sugerir cambiar status a `active`
+- `untracked_on_disk`: artefacto indexado con carpeta en disco pero sin fila en `pc_locations` → sugerir insertar