fix: emit real unified diff for remote workspace pending changes (#429)

[chubes4]

Summary

RemoteWorkspaceBackend::build_full_file_diff() was not a unified diff. It dumped the entire old content as - lines and the entire new content as + lines regardless of how small the actual change was. Agents calling workspace_git_diff to verify a surgical workspace_edit saw their whole file marked for removal and re-addition, and consistently misinterpreted this as "whole-file line-ending weather" — concluding the edit had nuked the file even though the actual write to GitHub was correct.

Real example: the world-of-wordpress agent hit this pattern three cycles in a row on the Observatory page in run 26050485830. A one-line <h2> heading change showed up as @@ -1,250 +1,278 @@ followed by every line removed then re-added, and the agent reverted its (correct) edit to avoid committing a noisy rewrite. The agent's restraint was correct given what it saw — but it was based on a phantom diff.

What this change does

Replaces the fake whole-file diff with a real unified-diff generator:

1. Common-prefix/suffix trimming on the line arrays, so the O(ND) core only runs on the actually-different middle window. Typical "surgical edit in large file" cases finish in O(N) instead of O(N^2).
2. Myers' O(ND) diff algorithm with V-array trace, walked backwards to reconstruct an ordered edit script.
3. Hunk grouping with three lines of context on each side, exactly the format git diff --no-color produces.
4. Proper @@ -start,count +start,count @@ markers, including the 0,0 edge cases for pure addition and pure deletion.

No new dependencies. ~280 lines of well-commented PHP added to a single private helper cluster.

Before vs after

Before (Observatory-like 250-line file with one change at line 125):

@@ -1,250 +1,251 @@
-line 1
-line 2
-line 3
...
-line 250
+line 1
+line 2
+line 3
...
+line 250

501 diff lines for a 1-line change. Agent panics.

After:

@@ -122,7 +122,8 @@
 line 122
 line 123
 line 124
-line 125
+replacement A
+replacement B
 line 126
 line 127
 line 128

7 diff lines, real context, agent sees the actual change.

Verification

Tested against seven representative cases:

Case	Result
two-line surgical (return 'old' → return 'new')	Single context line + one -/+ pair
identical content	Header only, no hunks
pure addition	@@ -0,0 +1,N @@ matching git
pure deletion	@@ -1,N +0,0 @@ matching git
middle change in 10-line file	Correct hunk with 3-line context
two separate hunks	Correctly split into two @@ blocks
observatory-like 250 lines	7-line hunk instead of 528-line phantom diff

tests/smoke-remote-workspace-backend.php passes 15/15 assertions:

• Existing diff assertion still matches (real unified diff still contains -return 'old'; and +return 'new';)
• New assertion: real hunk header @@ -1,2 +1,2 @@ is emitted
• New assertion: unchanged <?php line appears as space-prefixed context, not as a -/+ pair

homeboy lint --changed-since origin/main → 0 findings.
homeboy test --changed-since origin/main → passed.

Why this is upstream-first

The world creator's daily memory describes this as "line-ending weather" — a real perception bug that has already shaped three days of agent decisions. Per the site's "fix upstream first, never paper over" rule, the right move is to fix DMC so every consumer of workspace_git_diff sees real diffs, rather than papering it over in the world-creator bundle prompt with "ignore the diff, trust the edit return value."

AI assistance

• AI assistance: Yes
• Tool(s): Claude Code (Sonnet 4.5)
• Used for: Diagnosed the agent "line-ending weather" pattern from the world-creator transcript, traced it to build_full_file_diff, implemented Myers' algorithm in PHP, and verified the output against representative cases including the Observatory-sized file. Chris confirmed the upstream-fix-first approach over agent-side workarounds.

Closes #429.

[homeboy-ci]

Homeboy Results — data-machine-code

Lint

✅ lint — passed

ℹ️ Full options: homeboy docs commands/lint
ℹ️ Save lint baseline: homeboy lint data-machine-code --baseline
Deep dive: homeboy lint data-machine-code --changed-since 3bc3032

Test

✅ test — passed

ℹ️ Auto-fix lint issues: homeboy refactor data-machine-code --from lint --write
ℹ️ Collect coverage: homeboy test data-machine-code --coverage
ℹ️ Pass args to test runner: homeboy test -- [args]
ℹ️ Full options: homeboy docs commands/test
Deep dive: homeboy test data-machine-code --changed-since 3bc3032

Audit

✅ audit — passed

• duplication — 3 finding(s)
• intra-method-duplication — 2 finding(s)
• dead_code — 1 finding(s)
• test_coverage — 1 finding(s)
• Total: 7 finding(s)

Deep dive: homeboy audit data-machine-code --changed-since 3bc3032

Tooling versions

• Homeboy CLI: homeboy 0.182.0+bf5469d
• Extension: wordpress from https://github.com/Extra-Chill/homeboy-extensions
• Extension revision: 65942142
• Action: unknown@unknown