Add documentation for history editing feature

Document the -b/--edit-history flag and rebase editor format.
Covers commands (pick/edit/drop), role editing, and common workflows.

README.md

@@ -95,6 +95,89 @@ export EDITOR=nvim  # or vscode, nano, vim, emacs, etc.
 
 *Note: If neither `VISUAL` nor `EDITOR` is set, the harness will refuse to launch the editor and throw an error instead.*
 
+## 📜 History Editing
+
+If a conversation goes off track, you can surgically edit your conversation history using the interactive history editing mode. This allows you to rewind to a specific point, modify messages, or remove problematic exchanges without starting over.
+
+### Invoking History Editing
+
+Use the `-b` or `--edit-history` flag along with `-F` to specify your chat file:
+
+```bash
+./build/drharness -F conversation.json -b
+```
+
+*Note: The `-F` flag is required because editing requires an existing chat file.*
+
+### The Rebase Editor Format
+
+When invoked, the harness opens your configured editor with all messages in your conversation listed in order. Each line follows a git rebase-style format:
+
+```
+<command> <index> [<role>] <display_content>
+```
+
+**Commands:**
+- `pick`: Keep the message unchanged
+- `edit`: Modify the message content and/or role
+- `drop`: Delete the message (or simply remove the line)
+
+**Role:** (optional, only for `edit` commands)
+- `[user]`
+- `[assistant]`
+- `[tool]`
+
+**Display content:** The first line of the message (for reference only)
+
+Lines starting with `#` are comments and are ignored. Empty lines are treated as drops.
+
+### Editing Message Content and Roles
+
+When you mark a message with `edit`, the harness will open your editor again to let you modify the content. The edit prompt shows:
+- The previous message as context (for reference)
+- The `>8` separator
+- The message content you can edit
+
+You can change the role by modifying the role in the rebase line (e.g., `edit 2 [user]`).
+
+### Common Workflows
+
+**Editing a single message:**
+```
+pick 0 [user] Hello, can you help me?
+pick 1 [assistant] Sure! What do you need?
+edit 2 [assistant] I misunderstood. Let me try again.
+pick 3 [user] That's better, thanks!
+```
+Then, when editing message 2, you'll be prompted to enter the new content.
+
+**Dropping unwanted messages:**
+```
+pick 0 [user] Start the project
+drop 1 [assistant] Wrong approach
+pick 2 [user] Let me try a different way
+```
+Or simply delete the line:
+```
+pick 0 [user] Start the project
+# pick 1 [assistant] Wrong approach
+pick 2 [user] Let me try a different way
+```
+
+**Modifying message roles:**
+```
+pick 0 [user] Write a function
+edit 1 [user] I meant write a class instead
+```
+
+**Continuing a conversation after editing:**
+After saving and exiting the editor, your changes are persisted to the chat file. The program exits, allowing you to inspect the result. To continue the conversation:
+```bash
+./build/drharness -F conversation.json -m "Continue from where we left off"
+```
+
+*Note: The history editor uses strict validation. Any syntax error (invalid command, out-of-range index, invalid role) will cause the editor to abort with an aggregated error message. No partial changes are applied.*
+
 ## 🐳 Docker Isolation
 
 The harness can be compiled and run inside an isolated Alpine Linux container.