docs(skills): document fetch fallback chain and pinned target post

Add a reader-proxy tier after defuddle, stop skipping URLs that only fail a
plain classifier fetch, let a session pin an earlier draft as the target post,
and tighten Substack image candidate handling (dedupe, stable numbering,
reuse a resolved source post across a batch).

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

@@ -1,6 +1,6 @@
 ---
 name: mt-add-image
-description: Add an image URL to today's Hugo newsletter Bonus Images section with a detected or user-provided label. Use directly for a known image URL or when dispatched by mt-add-url.
+description: Add an image URL to the target Hugo newsletter post's Bonus Images section with a detected or user-provided label. Use directly for a known image URL or when dispatched by mt-add-url.
 ---
 
 # Add a newsletter image

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

@@ -1,6 +1,6 @@
 ---
 name: mt-add-post
-description: Add an article or blog URL to today's Hugo newsletter with its original title and a Vietnamese summary. Use directly for a known article URL or when dispatched by mt-add-url.
+description: Add an article or blog URL to the target Hugo newsletter post with its original title and a Vietnamese summary. Use directly for a known article URL or when dispatched by mt-add-url.
 ---
 
 # Add a newsletter article

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

@@ -1,6 +1,6 @@
 ---
 name: mt-add-video
-description: Add a YouTube URL to today's Hugo newsletter Bonus Videos section with a Vietnamese title and summary. Use directly for a known YouTube URL or when dispatched by mt-add-url.
+description: Add a YouTube URL to the target Hugo newsletter post's Bonus Videos section with a Vietnamese title and summary. Use directly for a known YouTube URL or when dispatched by mt-add-url.
 ---
 
 # Add a newsletter video

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

@@ -1,6 +1,6 @@
 ---
 name: mt-webfetch
-description: Fetch public web content through the defuddle fallback only after the built-in fetch is blocked or returns unusable content. Do not use for paywalls, login walls, or as the first fetch attempt.
+description: Fetch public web content through the fallback chain (defuddle, then a reader proxy) only after the built-in fetch is blocked or returns unusable content. Do not use for paywalls, login walls, or as the first fetch attempt.
 ---
 
 # Fetch blocked newsletter sources