feat(token): add stored token support with resolve_token and token_source

Introduce a centralized token resolution system that supports both
environment variables and config-file-stored tokens with clear priority
(env var wins). This enables cron-based sync which runs in minimal
shell environments without env vars.

Core changes:
- GitLabConfig gains optional `token` field and `resolve_token()` method
  that checks env var first, then config file, returning trimmed values
- `token_source()` returns human-readable provenance ("environment variable"
  or "config file") for diagnostics
- `ensure_config_permissions()` enforces 0600 on config files containing
  tokens (Unix only, no-op on other platforms)

New CLI commands:
- `lore token set [--token VALUE]` — validates against GitLab API, stores
  in config, enforces file permissions. Supports flag, stdin pipe, or
  interactive entry.
- `lore token show [--unmask]` — displays masked token with source label

Consumers updated to use resolve_token():
- auth_test: removes manual env var lookup
- doctor: shows token source in health check output
- ingest: uses centralized resolution

Includes 10 unit tests for resolve/source logic and 2 for mask_token.

src/cli/commands/doctor.rs

@@ -240,14 +240,14 @@ async fn check_gitlab(config: Option<&Config>) -> GitLabCheck {
         };
     };
 
-    let token = match std::env::var(&config.gitlab.token_env_var) {
-        Ok(t) if !t.trim().is_empty() => t.trim().to_string(),
-        _ => {
+    let token = match config.gitlab.resolve_token() {
+        Ok(t) => t,
+        Err(_) => {
             return GitLabCheck {
                 result: CheckResult {
                     status: CheckStatus::Error,
                     message: Some(format!(
-                        "{} not set in environment",
+                        "Token not set. Run 'lore token set' or export {}.",
                         config.gitlab.token_env_var
                     )),
                 },
@@ -257,6 +257,8 @@ async fn check_gitlab(config: Option<&Config>) -> GitLabCheck {
         }
     };
 
+    let source = config.gitlab.token_source().unwrap_or("unknown");
+
     let client = GitLabClient::new(&config.gitlab.base_url, &token, None);
 
     match client.get_current_user().await {
@@ -264,7 +266,7 @@ async fn check_gitlab(config: Option<&Config>) -> GitLabCheck {
             result: CheckResult {
                 status: CheckStatus::Ok,
                 message: Some(format!(
-                    "{} (authenticated as @{})",
+                    "{} (authenticated as @{}, token from {source})",
                     config.gitlab.base_url, user.username
                 )),
             },