feat(logging): unified log levels, Redact via LogValuer, CLI flag, settings migration

CLAUDE.md

@@ -85,4 +85,6 @@ KeyLint is a desktop app that fixes/enhances clipboard text via AI (OpenAI, Anth
 
 **Reference docs:** `.claude/docs/architecture.md` (service wiring, RPC bridge, platform differences), `.claude/docs/testing.md` (detailed patterns), `.claude/docs/versioning.md` (release pipeline, CI)
 
+**Logging conventions:** `docs/logging.md` (levels, Redact() usage, source tagging, CLI flags)
+
 **Pyramidize docs:** `docs/pyramidize/` (requirements, ADR, quality status, NLP/LangChain research, UX roadmap)

docs/logging.md

@@ -0,0 +1,51 @@
+# Logging Conventions
+
+## Log Levels
+
+| Level   | When to use                                    |
+|---------|------------------------------------------------|
+| off     | Default. No logging.                           |
+| trace   | Verbose internals, hot-path detail              |
+| debug   | Diagnostic info for developers                  |
+| info    | Normal operational events                       |
+| warning | Recoverable issues                              |
+| error   | Failures requiring attention                    |
+
+## Sensitive Redaction
+
+**Rule:** If a log value could contain user text, API payloads, or credentials, wrap it in `logger.Redact()`.
+
+```go
+// Safe metadata — no wrapping needed
+logger.Info("enhance: start", "provider", cfg.ActiveProvider, "input_len", len(text))
+
+// Sensitive data — wrap in Redact()
+logger.Debug("enhance: request", "provider", "openai", "payload", logger.Redact(string(body)))
+```
+
+When `SensitiveLogging` is off, `Redact()` outputs `[redacted]`. When on, the real value is shown. Uses slog's native `LogValuer` interface.
+
+Never wrap: provider names, status codes, byte lengths, error messages, config keys.
+Always wrap: API request/response bodies, user text, clipboard content, API keys.
+
+## Source Tagging
+
+All log entries include a `source` attribute:
+- `source=backend` — Go backend (automatic via default logger instance)
+- `source=frontend` — Angular frontend (via the log bridge service)
+
+## CLI Usage
+
+```bash
+./bin/KeyLint -fix --log debug "text to fix"
+./bin/KeyLint -pyramidize --log info -f input.md
+```
+
+Valid levels: `off`, `trace`, `debug`, `info`, `warning`, `error`. Default: `off`.
+Sensitive logging is always off in CLI mode to prevent credential leaks to terminal.
+
+## Settings
+
+- **UI:** Settings > General > Log Level dropdown (Off/Trace/Debug/Info/Warning/Error)
+- **JSON field:** `"log_level": "off"` (replaces legacy `"debug_logging": true/false`)
+- **Migration:** Legacy `debug_logging: true` is auto-migrated to `log_level: "debug"` on load