feat: add analytics subcommand for mbox sender analysis

Adds a new `analytics` subcommand that analyzes Google Takeout mbox files
to identify top senders by message count. Designed for efficient processing
of large files (60GB+) with minimal memory usage.

Features:
- Streams files line-by-line with 1MB buffer (never loads entire file)
- Extracts sender email addresses from From: headers
- Counts messages per sender and displays top N (default 10)
- Shows progress output every 10,000 messages
- No Gmail API access needed

Usage:
  cull-gmail analytics <MBOX_FILE> [-n TOP]

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

README.md

@@ -53,8 +53,9 @@ Get started with cull-gmail in minutes using the built-in setup command:
 - **Flexible configuration**: Support for file-based config, environment variables, and ephemeral tokens
 - **Safety first**: Dry-run mode by default, interactive confirmations, and timestamped backups
 - **Label management**: List and inspect Gmail labels for rule planning
-- **Message operations**: Query, filter, and perform batch operations on Gmail messages  
+- **Message operations**: Query, filter, and perform batch operations on Gmail messages
 - **Rule-based automation**: Configure retention rules with time-based filtering and automated actions
+- **Mbox analysis**: Analyze Google Takeout exports to identify top senders (efficient streaming, no API needed)
 - **Token portability**: Export/import OAuth2 tokens for containerized and CI/CD environments
 
 ### Running the optional Gmail integration test
@@ -201,9 +202,12 @@ cull-gmail [OPTIONS] [COMMAND]
 
 ### Commands
 
+- `init`: Initialize configuration and OAuth2 credentials
 - `labels`: List available Gmail labels
 - `messages`: Query and operate on messages
 - `rules`: Configure and run retention rules
+- `analytics`: Analyze mbox files for sender statistics
+- `token`: Export and import OAuth2 tokens
 
 ## Command Reference
 
@@ -370,6 +374,70 @@ cull-gmail rules run --execute --skip-trash
 cull-gmail rules run --execute --skip-delete
 ```
 
+### Analytics Command
+
+Analyze Google Takeout mbox files to identify top senders by message count.
+
+**Note**: This command does NOT require Gmail API access. It efficiently streams local mbox files with minimal memory usage, making it suitable for analyzing large exports (60GB+).
+
+#### Syntax
+
+```bash
+cull-gmail analytics [OPTIONS] <MBOX_FILE>
+```
+
+#### Arguments
+
+- `<MBOX_FILE>`: Path to mbox file to analyze (typically from Google Takeout)
+
+#### Options
+
+- `-n, --top <TOP>`: Number of top senders to display [default: 10]
+
+#### Examples
+
+**Show top 10 senders from a Google Takeout mbox**:
+```bash
+cull-gmail analytics ~/takeout/All\ mail\ Including\ Spam\ and\ Trash.mbox
+```
+
+**Show top 20 senders**:
+```bash
+cull-gmail analytics -n 20 ~/takeout/All\ mail.mbox
+```
+
+**Example Output**:
+```
+[INFO] Scanned 1234567 messages total.
+Top 10 senders:
+    45678  newsletter@example.com
+    23456  promotions@example.com
+    18901  notifications@example.com
+    12345  support@example.com
+     9876  marketing@example.com
+     8765  updates@example.com
+     7654  alerts@example.com
+     6543  digests@example.com
+     5432  reports@example.com
+     4321  announcements@example.com
+```
+
+#### Use Cases
+
+- Identify top email senders in your mailbox before configuring rules
+- Analyze historical email patterns from a full account export
+- Find unexpected high-volume senders for further investigation
+- Plan email retention policies based on actual sender frequency
+
+#### Getting a Google Takeout mbox File
+
+1. Visit [Google Takeout](https://takeout.google.com)
+2. Select "Gmail" and choose the desired email account
+3. Select export format "Standard" (generates .mbox files)
+4. Download the export (can be very large - multiple parts possible)
+5. Extract/combine the mbox files if needed
+6. Use `cull-gmail analytics` on the mbox file
+
 ## Gmail Query Syntax
 
 The `-Q, --query` option supports Gmail's powerful search syntax: