fix(hashline-edit): improve error messages for invalid LINE#ID references

- Detect non-numeric prefixes (e.g., "LINE#HK", "POS#VK") and explain
  that the prefix must be an actual line number, not literal text
- Add suggestLineForHash() that reverse-looks up a hash in file lines
  to suggest the correct reference (e.g., Did you mean "1#HK"?)
- Unify error message format from "LINE#ID" to "{line_number}#{hash_id}"
  matching the tool description convention
- Add 3 tests covering non-numeric prefix detection and hash suggestion

src/tools/hashline-edit/validation.test.ts

@@ -19,7 +19,23 @@ describe("parseLineRef", () => {
     const ref = "42:VK"
 
     //#when / #then
-    expect(() => parseLineRef(ref)).toThrow("LINE#ID")
+    expect(() => parseLineRef(ref)).toThrow("{line_number}#{hash_id}")
+  })
+
+  it("gives specific hint when literal text is used instead of line number", () => {
+    //#given — model sends "LINE#HK" instead of "1#HK"
+    const ref = "LINE#HK"
+
+    //#when / #then — error should mention that LINE is not a valid number
+    expect(() => parseLineRef(ref)).toThrow(/not a line number/i)
+  })
+
+  it("gives specific hint for other non-numeric prefixes like POS#VK", () => {
+    //#given
+    const ref = "POS#VK"
+
+    //#when / #then
+    expect(() => parseLineRef(ref)).toThrow(/not a line number/i)
   })
 
   it("accepts refs copied with markers and trailing content", () => {
@@ -60,4 +76,13 @@ describe("validateLineRef", () => {
     expect(() => validateLineRefs(lines, ["2#ZZ"]))
       .toThrow(/>>>\s+2#[ZPMQVRWSNKTXJBYH]{2}\|two/)
   })
+
+  it("suggests correct line number when hash matches a file line", () => {
+    //#given — model sends LINE#XX where XX is the actual hash for line 1
+    const lines = ["function hello() {", "  return 42", "}"]
+    const hash = computeLineHash(1, lines[0])
+
+    //#when / #then — error should suggest the correct reference
+    expect(() => validateLineRefs(lines, [`LINE#${hash}`])).toThrow(new RegExp(`1#${hash}`))
+  })
 })