refactor(hashline-edit): stabilize hashes and tighten prefix stripping

Switch line hashing to significance-aware seeding so meaningful lines stay stable across reflows while punctuation-only lines still disambiguate by line index.

Also narrow prefix stripping to hashline/diff patterns that reduce accidental content corruption during edit normalization.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/hashline-edit/hash-computation.test.ts

@@ -21,7 +21,7 @@ describe("computeLineHash", () => {
     expect(hash1).toMatch(/^[ZPMQVRWSNKTXJBYH]{2}$/)
   })
 
-  it("produces different hashes for same content on different lines", () => {
+  it("produces same hashes for significant content on different lines", () => {
     //#given
     const content = "function hello() {"
 
@@ -29,6 +29,18 @@ describe("computeLineHash", () => {
     const hash1 = computeLineHash(1, content)
     const hash2 = computeLineHash(2, content)
 
+    //#then
+    expect(hash1).toBe(hash2)
+  })
+
+  it("mixes line number for non-significant lines", () => {
+    //#given
+    const punctuationOnly = "{}"
+
+    //#when
+    const hash1 = computeLineHash(1, punctuationOnly)
+    const hash2 = computeLineHash(2, punctuationOnly)
+
     //#then
     expect(hash1).not.toBe(hash2)
   })