feat(skill): auto-save research results to ~/Documents/Last30Days/

Every run now automatically saves the complete briefing (synthesis,
stats, follow-up suggestions) as a topic-named .md file in the user's
Documents folder. Agent mode also saves. No Python script changes -
this is purely a SKILL.md instruction addition.

Inspired by @devin_explores manually saving results to build a
personal research library.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SKILL.md

@@ -123,6 +123,8 @@ If `--agent` appears in ARGUMENTS (e.g., `/last30days plaud granola --agent`):
 5. **Skip** the follow-up invitation ("I'm now an expert on X...")
 6. **Output** the complete research report and stop - do not wait for further input
 
+Agent mode still saves the research briefing to `~/Documents/Last30Days/` using the same logic as interactive mode (see "Save Research to Documents" section).
+
 Agent mode report format:
 
 ```
@@ -496,6 +498,51 @@ For `/last30days war in Iran` (NEWS):
 
 ---
 
+## Save Research to Documents
+
+After displaying the invitation, save the complete research briefing to the user's Documents folder. This happens automatically on every run.
+
+1. **Create the directory** (if it doesn't exist):
+   ```bash
+   mkdir -p ~/Documents/Last30Days
+   ```
+
+2. **Generate the filename** from TOPIC:
+   - Lowercase the topic
+   - Replace spaces and special characters with hyphens
+   - Remove consecutive hyphens
+   - Trim to 60 characters max
+   - Example: "Claude Code Best Practices" -> `claude-code-best-practices`
+
+3. **Check for duplicates**: If `~/Documents/Last30Days/{slug}.md` already exists, append today's date: `{slug}-YYYY-MM-DD.md`
+
+4. **Use the Write tool** to save to `~/Documents/Last30Days/{slug}.md` with this exact structure:
+
+```markdown
+# {TOPIC}
+
+> Researched {date} | Query type: {QUERY_TYPE} | Target tool: {TARGET_TOOL or "general"}
+
+## What I learned
+
+{The full synthesis section you just displayed - all topics, patterns, and citations}
+
+## Stats
+
+{The full stats box with source counts and engagement - copy exactly as displayed}
+
+## Follow-up suggestions
+
+{The 2-3 specific suggestions from the invitation block}
+
+---
+*Generated by [last30days](https://github.com/mvanhorn/last30days-skill) v2.9*
+```
+
+5. **Confirm briefly** after saving: `Saved to ~/Documents/Last30Days/{slug}.md`
+
+---
+
 ## WAIT FOR USER'S RESPONSE
 
 After showing the stats summary with your invitation, **STOP and wait** for the user to respond.
@@ -607,6 +654,7 @@ Want another prompt? Just tell me what you're creating next.
 - Optionally sends search queries to Brave Search API, Parallel AI API, or OpenRouter API for web search
 - Fetches public Reddit thread data from `reddit.com` for engagement metrics
 - Stores research findings in local SQLite database (watchlist mode only)
+- Saves research briefings as .md files to ~/Documents/Last30Days/
 
 **What this skill does NOT do:**
 - Does not post, like, or modify content on any platform