feat(hashline): added full-file hash snapshots with 4-hex tags

- Replaced snapshot internals with full-file records and removed contiguous/sparse snapshot APIs.
- Added file-hash normalization, computed `computeFileHash`, and updated grammar/messages to 4-hex tags.
- Simplified recovery by checking whole-file hashes first, then applying merge-replay fallback after mismatches.
- Updated coding-agent tools to use `record`/`recordFileSnapshot` and skip hash headers for unsnapshotted large files.
- Expanded patcher and snapshot tests to verify 4-hex anchors, hash deduplication, and cache-capped behavior.

packages/coding-agent/test/edit-diff.test.ts

@@ -240,7 +240,7 @@ describe("computeHashlineDiff", () => {
 		// fires through computeHashlineDiff but produces identical content.
 		const text = `${line}\n`;
 		const snapshotStore = new InMemorySnapshotStore();
-		const tag = snapshotStore.recordContiguous(sourcePath, 1, text.split("\n"), { fullText: text });
+		const tag = snapshotStore.record(sourcePath, text);
 		const input = `${formatHashlineHeader(sourcePath, tag)}\nreplace 1..1:\n+${line}\n`;
 		const result = await computeHashlineDiff({ input }, tempDir, snapshotStore);
 		expect("error" in result).toBe(true);