Introduce Diffy v0.1.0: context-menu git diffing for VS Code

[MelbourneDeveloper]

TLDR

Initial implementation of the Diffy VS Code extension — pick two git states (commit / branch / tag / index / working copy) from existing SCM/editor/explorer context menus and diff them through `vscode.diff`, with the recent fix that the RefPicker now hides the current branch.

Details

Extension surface (`package.json`, `src/menus.ts`, `src/extension.ts`) — seven commands wired exclusively into menus VS Code already has: `scm/historyItem/context` (compareWith, compareWithWorkingCopy, compareWithPrevious, compareWithBranch, compareWithTag), `scm/resourceState/context` + `editor/title/context` + `explorer/context` (compareFileWithCommit / Branch / Tag), and the Command Palette (compareTwoCommits, compareFileWithCommit, reopenLast). No new views, sidebars, activity-bar icons, or webviews — `src/menus.ts` is the single source of truth and `scripts/sync-menus.mjs` keeps `package.json` byte-identical with a check mode for CI.

Git layer (`src/git/`, zero `vscode` imports) — `GitRunner` shells out to `git` via `child_process.spawn` returning `Result<string, GitError>`. `GitRepo` exposes `log`, `nameStatus`, `numstat`, `show`, `refs`, `revParse`, and `currentBranch` (the latter added in this branch via `git branch --show-current` to drive the new exclude-current-branch behavior). All porcelain parsing is NUL-delimited (`-z` everywhere) in `src/git/parsers.ts` — no regex over structured git output. Multi-root workspaces resolve via longest-prefix match in `src/git/repoMatch.ts`.

UI (`src/ui/`) — thin wrappers around `vscode.window.createQuickPick()` returning `Result<T, Cancelled>`. `FilePicker` stays open after selection (`ignoreFocusOut: true`) so a single comparison can drive many diffs. `RefPicker` accepts an optional `excludeBranchName` that is filtered out by the pure `src/ui/format/refFilter.ts` module — branch-only exclusion, so a tag with the same name is kept.

Compare flow (`src/commands/flow.ts`) — `pickRefAsSha` now looks up `repo.currentBranch()` and threads the name into `pickRef` so the user's own branch is hidden from the picker (a current-branch lookup failure logs and degrades silently rather than blocking the picker). `drillIntoFiles` records the comparison via `MementoStore.setLastComparison` then opens `FilePicker` to fan out per-file `vscode.diff` invocations.

Content provider (`src/providers/DiffyContentProvider.ts`) — implements `TextDocumentContentProvider` for scheme `diffy://`; URI codec and parse errors live in pure `src/ui/uri.ts` (`buildDiffyUri` / `parseDiffyUri`, round-tripping unicode, spaces, `#`, `?`).

Repo plumbing — `Makefile` exposes the 7 standard targets (`build`, `test`, `lint`, `fmt`, `clean`, `ci`, `setup`) plus repo-specific `package`; `test:coverage` is fail-fast, runs unit + E2E, enforces threshold from `coverage-thresholds.json` (default 95). TS in strict mode with `exactOptionalPropertyTypes`; ESLint + Prettier; pino logger writing to file + VS Code OutputChannel via `src/logger.ts`. `shipwright.json` + `schemas/shipwright.schema.json` define release metadata, validated by `scripts/validate-shipwright-manifest.mjs`. CI runs `.github/workflows/ci.yml`, release builds via `.github/workflows/release.yml`.

Test fixtures — `test-fixtures/repo-seed/seed.sh` builds a deterministic three-commit repo with a `feature` branch at commit 2 and a `v0.1.0` tag at commit 2, so the RefPicker E2E tests have a non-current branch to pick after `main` is hidden.

Brand assets — primary icon set at 128/256/512/1024 PNG + WebP under `docs/assets/diffy-icon-primary-*`; the root `icon.png` is byte-identical to the 128 px export (verified by sha1 in this branch's docs change). `docs/design-system.md` Logo Direction table pruned of contact-sheet / brackets / panes / delta / gutter rows that referenced files that no longer ship.

How Do The Automated Tests Prove It Works?

`make ci` is green on this branch: 141 unit + 35 E2E tests pass, coverage at 95.62 % statements / 87.34 % branches / 97.82 % functions / 95.62 % lines — above the 95 threshold in `coverage-thresholds.json`.

Spot-check coverage of the recent behavior added in this branch:

• `src/test/unit/refFilter.test.ts` (7 tests, 100 % file coverage) — `filterRefs` returns refs untouched with no filters; `type=branch` / `type=tag` keep only the matching ref kind; `excludeBranchName='agentpmo'` drops the branch but keeps the same-named tag (this is the exact scenario the user reported); branch-typed filter + exclude name returns the remaining branches; `excludeBranchName=undefined` is a no-op (detached-HEAD case); excluding a name never removes tags.
• `src/test/suite/commands.test.ts → compareWithBranch(historyItem={id:sha1}) → branch-filtered RefPicker hides current branch `main` → diff vs feature(=sha2)` — drives the SCM-history command end-to-end via `vscode.commands.executeCommand` and asserts the resulting `vscode.TabInputTextDiff` has `diffy://commit/${sha1}/` on the left, `diffy://commit/${sha2}/` on the right, and the human-readable label ` ↔ — …`.
• `compareFileWithBranch(uri:a.txt) → branch RefPicker hides current branch → diff opens for feature(=sha2) vs working copy` — same shape against the editor-tab command; left is `diffy://commit/${sha2}/a.txt`, right is the on-disk `file://` URI, label is ` ↔ Working Copy — a.txt`.
• `compareWith → SideB=pickRef → user picks the v0.1.0 tag → diff against that ref` — confirms the picker order is now (alphabetical, current-branch hidden) `feature`, `v0.1.0`.
• Parser suite (`src/test/unit/parsers.test.ts`, 41 tests) — every `A/M/D/R/C` status, numstat binary marker (`-`/`-`), empty input, malformed inputs → `Err`, unicode + quoted paths.
• URI round-trip (`src/test/unit/uri.test.ts`, 23 tests) — spaces, unicode, `#`, `?`, deeply nested paths.
• Menu manifest (`src/test/unit/menus.test.ts`, 8 tests) — `package.json` `contributes.menus` and `contributes.commands` are byte-equal to what `src/menus.ts` would write; every `COMMAND_ID` has a title; SCM history menu contains all five commit-level commands; the proposed `contribSourceControlHistoryItemMenu` API is declared.

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