refactor(scripts): migrate newsletter engine from Node to Go

One Go binary with a subcommand per former script, invoked as
'go run ./scripts/newsletter <command>' from repo root. Stdlib only;
publications config embedded via go:embed (go run recompiles on edit).
Behavior parity verified side-by-side before deleting the JS —
see plans/reports/parity-260818-newsletter-go-migration-report.md.

Permissions: allow 'Bash(go *)', drop 'Bash(node *)' and the redundant
'Write(content)' rule (Edit rules cover all file-editing tools).

.claude/skills/mt-add-image/SKILL.md

@@ -21,24 +21,24 @@ A clean image URL (passed by `mt-add-url`, or given directly).
 
 ### 1. Detect source
 ```bash
-node scripts/newsletter/detect-image-source.js "<url>"
+go run ./scripts/newsletter detect-image-source "<url>"
 ```
 → `{ original_url, clean_url, isSubstack, uuid?, innerUrl? }`.
 
 When invoked **directly** (not via `mt-add-url`), first run the router to get accessibility + duplicate status and skip accordingly:
 ```bash
-node scripts/newsletter/add-url.js "<url>"   # expect route:image; skip if duplicate/!accessible
+go run ./scripts/newsletter add-url "<url>"   # expect route:image; skip if duplicate/!accessible
 ```
 (When dispatched by `mt-add-url`, that check already ran — don't repeat it.)
 
 ### 2a. Substack image (`isSubstack: true` with `uuid`)
 Find the source post:
 ```bash
-node scripts/newsletter/find-substack-post.js --uuid <uuid>
+go run ./scripts/newsletter find-substack-post --uuid <uuid>
 ```
 - `found: false` → retry with the deeper sitemap crawl (slower — fetches posts ~3 months back, capped at 40 fetches total across all publications; warn the user it may take a while):
   ```bash
-  node scripts/newsletter/find-substack-post.js --uuid <uuid> --deep
+  go run ./scripts/newsletter find-substack-post --uuid <uuid> --deep
   ```
   On a miss the result reports `scanned` (posts fetched), `budget` (the 40-fetch cap), and `cutoff` (oldest date looked at) — mention how far back it looked.
 - `found: false` after `--deep` → no source post; go to step 3 (ask) and/or step 4 (add publication).

.claude/skills/mt-add-post/SKILL.md

@@ -13,7 +13,7 @@ Shared scripts: `scripts/newsletter/`. Shared procedure: `../mt-add-url/referenc
 
 A clean article URL (passed by `mt-add-url`, or given directly). If a raw URL is provided directly, you may run the classifier to clean/dedup it first:
 ```bash
-node scripts/newsletter/add-url.js "<url>"
+go run ./scripts/newsletter add-url "<url>"
 ```
 Trust `route: article`; skip if `duplicate` or not `accessible`.
 

.claude/skills/mt-add-tags/SKILL.md

@@ -50,7 +50,7 @@ Read the full post body. Identify:
 
 Before generating, run:
 ```bash
-node scripts/newsletter/list-existing-tags.js
+go run ./scripts/newsletter list-existing-tags
 ```
 When a proposed tag matches an existing one case-insensitively, use the existing casing.
 -->

.claude/skills/mt-add-url/SKILL.md

@@ -20,7 +20,7 @@ Everything else (direct `video` file, `document`, or anything unrecognized) is *
 
 For every URL the user provides:
 ```bash
-node scripts/newsletter/add-url.js "<url>"
+go run ./scripts/newsletter add-url "<url>"
 ```
 Output (JSON): `{ original_url, clean_url, http_status, accessible, duplicate, route, title?, author? }`.
 

.claude/skills/mt-add-url/references/newsletter-post-mechanics.md

@@ -18,7 +18,7 @@ Get current date in `YYYY-MM-DD` (UTC+7). Check `content/post/YYYY/MM/DD/index.m
 ## 2. Newsletter number
 
 ```bash
-node scripts/newsletter/find-newsletter-number.js
+go run ./scripts/newsletter find-newsletter-number
 ```
 Searches backwards from today for the most recent newsletter and returns the next number. Only needed when **creating** a new post.
 

.claude/skills/mt-add-video/SKILL.md

@@ -19,7 +19,7 @@ A clean YouTube URL (passed by `mt-add-url`, or given directly).
 
 1. **Classify / fetch title** — run the router to get the canonical URL + title:
    ```bash
-   node scripts/newsletter/add-url.js "<url>"
+   go run ./scripts/newsletter add-url "<url>"
    ```
    Confirm `route: youtube`; skip if `duplicate` or not `accessible`. Use the returned `clean_url` (canonical `watch?v=ID`) and `title`.
    - If `title` is missing (oEmbed failed), fetch the title via WebFetch on the watch URL.

.claude/skills/mt-webfetch/SKILL.md

@@ -28,7 +28,7 @@ Use this skill only after a WebFetch attempt returned one of:
 1. Confirm WebFetch already failed on the target URL
 2. Run the fetch script:
    ```bash
-   node scripts/newsletter/fetch-via-defuddle.js "<target_url>"
+   go run ./scripts/newsletter fetch-via-defuddle "<target_url>"
    ```
    Alternatively, use WebFetch with the defuddle-prefixed URL:
    ```
@@ -70,7 +70,7 @@ Never loop. Never retry more than once.
 User wanted to extract content from https://example.com/article
 WebFetch returned: "Request failed with status code 403"
 → Trigger mt-webfetch
-→ node scripts/newsletter/fetch-via-defuddle.js "https://example.com/article"
+→ go run ./scripts/newsletter fetch-via-defuddle "https://example.com/article"
 → Parse markdown output
 → Summarize as usual
 ```