feat: Sprint 15 — session projects, code copy button, tool card toggle

Session projects: named groups for organizing sessions. Project filter
bar with chips between search and session list. Create/rename/delete
projects, assign sessions via folder icon dropdown. Stored in
projects.json, project_id on Session model. 5 new API endpoints.

Code block copy button: every code block gets a Copy button in the
language header (or top-right for plain blocks). Clipboard API with
"Copied!" feedback.

Tool card expand/collapse: messages with 2+ tool cards get an
"Expand all / Collapse all" toggle above the card group.

13 new tests (237 total), all passing. No regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/config.py

@@ -39,6 +39,7 @@ WORKSPACES_FILE       = STATE_DIR / 'workspaces.json'
 SESSION_INDEX_FILE    = SESSION_DIR / '_index.json'
 SETTINGS_FILE         = STATE_DIR / 'settings.json'
 LAST_WORKSPACE_FILE   = STATE_DIR / 'last_workspace.txt'
+PROJECTS_FILE         = STATE_DIR / 'projects.json'
 
 # ── Hermes agent directory discovery ─────────────────────────────────────────
 def _discover_agent_dir() -> Path: