docs(coding-agent/prompts): clarified hashline insert and line-number rules

- Added rule that inserts add only listed rows; trailing blank payload writes a literal empty line.
- Added rule that a bare anchor op inserts one blank line and is not a no-op.
- Clarified that frozen line numbers do not shift as earlier ops in the same hunk apply.
- Expanded anti-pattern example to show trailing-blank doubling and anchor-offset mistakes.

packages/coding-agent/src/prompts/tools/hashline.md

@@ -19,8 +19,10 @@ A-B!           delete A..B   (or A! == A..A)
 <rules>
 - **Payload is only what's NEW.** `:` replaces inside; `↑`/`↓` add at anchor. NEVER repeat anchor lines or neighbors.
 - **Continuation lines require `+`.** Use `+` for a blank payload line; use `++text` to write a line starting with `+text`.
+- **Inserts add ONLY the rows you list.** The file's existing newlines around the anchor stay. NEVER tack a trailing `+` blank "for spacing" — it writes a literal blank line into the file, doubling whatever is already there.
+- **A bare `LINE↑`/`LINE↓` with no payload still inserts ONE blank line.** Not a no-op. Omit the op if you want nothing there.
 - **Go small.** Add → `↑`/`↓`; replace → `:`; delete → `!`.
-- **Line numbers are frozen references to what you have seen.** Later ops still use original line numbers.
+- **Line numbers are frozen references to what you have seen.** Later ops in the same hunk still use original line numbers; they do NOT shift as earlier ops apply.
 </rules>
 
 <common-failures>
@@ -32,7 +34,9 @@ A-B!           delete A..B   (or A! == A..A)
 <example>
 ```a.ts#1a2b
 1:const X = "a";
-2:export function f() { return X; }
+2:
+3:export function f() { return X; }
+4:f();
 ```
 
 # replace with a continuation line, insert after, delete
@@ -41,7 +45,7 @@ A-B!           delete A..B   (or A! == A..A)
 1:const X = "b";
 +export const Y = X;
 1↓const Z = Y;
-2!
+4!
 ```
 </example>
 
@@ -52,6 +56,12 @@ A-B!           delete A..B   (or A! == A..A)
 1:const X = "a";
 1-2:const X = "b";
 +export const Y = X;
+# WRONG — trailing `+` blank writes a literal empty line; the new blank lands right next to the orig blank at line 2, doubling it
+1↓const Y = X;
++
+# WRONG — `2↓` still anchors at PRE-EDIT line 2 (frozen), NOT at the line just inserted by `1↓`. Both inserts land at their own anchors, giving three consecutive blanks (new from `1↓`, orig blank line 2, new from `2↓`).
+1↓
+2↓
 </anti-pattern>
 
 <critical>