feat(workspace): multi-scope workspace reads (#1117)

* feat(workspace): multi-scope workspace reads

Adds the ability for a workspace to read from multiple user scopes
while keeping writes isolated to the primary scope. Configuration
via WORKSPACE_READ_SCOPES env var (comma-separated user IDs).

Includes identity file isolation (read_primary), multi-scope search,
list, and read operations, WorkspaceConfig refactor, and comprehensive
integration tests.

* fix: address review feedback for multi-scope workspace reads

- fix(memory): deduplicate timezone parsing for daily_log target
  parse_timezone was called twice when target was "daily_log" without a
  layer — once in path resolution, again in the fallback. Now computed
  once and reused.

- fix(config): add character validation for WORKSPACE_READ_SCOPES and
  layer scopes — both enforce [a-zA-Z0-9_-] to prevent path traversal
  or injection via scope strings used as user_id in SQL queries.

- fix(config): use chars().take(32) instead of byte-index slicing for
  scope length error messages (UTF-8 safety).

- fix(error): remove unused WorkspaceError::NotFound variant

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* style: downgrade search log to debug, add comments on list iteration

- Downgrade hybrid_search_multi tracing::info! to debug! — fires on
  every multi-scope search with the default backend, too noisy for info
- Add comments explaining why list/list_all iterate per-scope instead
  of using _multi trait methods (identity path filtering needs scope
  attribution that merged results lose)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: [email protected] <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

src/tools/builtin/memory.rs

@@ -271,12 +271,13 @@ impl Tool for MemoryWriteTool {
             .and_then(|v| v.as_bool())
             .unwrap_or(false);
 
+        // Parse timezone once for targets that need it (daily_log).
+        let tz = crate::timezone::parse_timezone(&ctx.user_timezone).unwrap_or(chrono_tz::Tz::UTC);
+
         // Resolve the target to a workspace path
         let resolved_path = match target {
             "memory" => paths::MEMORY.to_string(),
             "daily_log" => {
-                let tz = crate::timezone::parse_timezone(&ctx.user_timezone)
-                    .unwrap_or(chrono_tz::Tz::UTC);
                 let now = chrono::Utc::now().with_timezone(&tz);
                 format!("daily/{}.md", now.format("%Y-%m-%d"))
             }
@@ -318,8 +319,6 @@ impl Tool for MemoryWriteTool {
                     }
                 }
                 "daily_log" => {
-                    let tz = crate::timezone::parse_timezone(&ctx.user_timezone)
-                        .unwrap_or(chrono_tz::Tz::UTC);
                     self.workspace
                         .append_daily_log_tz(content, tz)
                         .await