Add rename-prefix command (bd-420)

- Implement bd rename-prefix command with --dry-run and --json flags
- Add prefix validation (max 8 chars, lowercase, starts with letter)
- Update all issue IDs and text references atomically per issue
- Update dependencies, labels, events, and counters
- Fix counter merge to use MAX() to prevent ID collisions
- Update snapshot tables for FK integrity
- Add comprehensive tests for validation and rename workflow
- Document in README.md and AGENTS.md

Known limitation: Each issue updates in its own transaction.
A failure mid-way could leave mixed state. Acceptable for
intended use case (infrequent operation on small DBs).

Amp-Thread-ID: https://ampcode.com/threads/T-7e77b779-bd88-44f2-9f0b-a9f2ccd54d38
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -311,6 +311,49 @@ bd update bd-1 --status in_progress --json
 bd close bd-1 --json
 ```
 
+### Renaming Prefix
+
+Change the issue prefix for all issues in your database. This is useful if your prefix is too long or you want to standardize naming.
+
+```bash
+# Preview changes without applying
+bd rename-prefix kw- --dry-run
+
+# Rename from current prefix to new prefix
+bd rename-prefix kw-
+
+# JSON output
+bd rename-prefix kw- --json
+```
+
+The rename operation:
+- Updates all issue IDs (e.g., `knowledge-work-1` → `kw-1`)
+- Updates all text references in titles, descriptions, design notes, etc.
+- Updates dependencies and labels
+- Updates the counter table and config
+
+**Prefix validation rules:**
+- Max length: 8 characters
+- Allowed characters: lowercase letters, numbers, hyphens
+- Must start with a letter
+- Must end with a hyphen (or will be trimmed to add one)
+- Cannot be empty or just a hyphen
+
+Example workflow:
+```bash
+# You have issues like knowledge-work-1, knowledge-work-2, etc.
+bd list  # Shows knowledge-work-* issues
+
+# Preview the rename
+bd rename-prefix kw- --dry-run
+
+# Apply the rename
+bd rename-prefix kw-
+
+# Now you have kw-1, kw-2, etc.
+bd list  # Shows kw-* issues
+```
+
 ### Dependencies
 
 ```bash