feat: add session persistence to CLI

- Add save-only endpoint (POST /conversations/{id}/save) for persisting
  messages without triggering agent execution
- Add sessions command to list previous conversation sessions
- Add --resume flag to chat command for resuming sessions by ID
- Buffer streamed responses and save after completion
- Update AGENTS.md with session commands and remove outdated limitation
- Add 3 new tests for save endpoint (208 total)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -88,13 +88,29 @@ cd webber-cli
 # Check API connection
 .venv/bin/webber-cli status
 
-# Explore a directory
-.venv/bin/webber-cli explore "find all python files" -d ../webber-sandbox
-
-# Interactive chat mode
+# Interactive chat (default mode - full capabilities)
 .venv/bin/webber-cli chat -d ../webber-sandbox
+
+# Read-only mode (safe exploration and planning)
+.venv/bin/webber-cli chat --mode plan -d ../webber-sandbox
+
+# Auto-accept mode (no approval prompts - use with caution)
+.venv/bin/webber-cli chat --mode auto_accept -d ../webber-sandbox
+
+# List previous sessions
+.venv/bin/webber-cli sessions
+
+# Resume a previous session
+.venv/bin/webber-cli chat --resume <session-id>
 ```
 
+**CLI Features:**
+- **Tab completion** for commands and file paths
+- **Command history** persisted to `~/.webber_history`
+- **Session persistence** - conversations saved and resumable
+- **Runtime mode switching** via `mode plan|default|auto_accept`
+- **Directory navigation** via `cd <path>`
+
 **Note:** The API server must be running for CLI commands to work.
 
 ---
@@ -153,9 +169,10 @@ pytest tests/ -v
 # 1. Load the template
 ./sandbox.sh load calculator-cli
 
-# 2. Have Webber explore it
+# 2. Have Webber explore it (plan mode = read-only)
 cd webber-cli
-.venv/bin/webber-cli explore "find all bugs in the code" -d ../webber-sandbox
+.venv/bin/webber-cli chat --mode plan -d ../webber-sandbox
+# Then ask: "find all bugs in the code"
 
 # 3. Check TASKS.md for expected bugs
 cat ../webber-sandbox/TASKS.md
@@ -258,7 +275,5 @@ cd webber-api
 ## Known Limitations
 
 1. **Model hallucination** - Mistral Nemo sometimes makes up file contents instead of using tool results
-2. **No conversation memory** - CLI chat mode doesn't persist between sessions
-3. **No streaming** - Responses appear all at once
 
 See `webber-api/docs/COVERAGE.md` for full feature coverage status.