fix(worktree): resolve Git executable outside PATH

[Priyanchew]

Summary

• Resolve Git via the shared platform layer before worktree operations.
• Fall back to common Git install locations on Windows, macOS, and Linux when PATH is degraded.
• Surface a clearer worktree setup error when Git is truly unavailable.

closes #1777

Test plan

• pnpm --filter @aoagents/ao-core test -- src/__tests__/platform.test.ts
• pnpm --filter @aoagents/ao-core typecheck && pnpm --filter @aoagents/ao-core build && pnpm --filter @aoagents/ao-plugin-workspace-worktree typecheck && pnpm --filter @aoagents/ao-plugin-workspace-worktree build
• Reduced-PATH repro: getGitExecutable() resolves C:\Program Files\Git\cmd\git.exe and workspaceExists=true

[github-actions]

Test Coverage Report

Metric	Value
Lines covered	75/76
Lines not covered	1/76
Overall coverage	98.7%

Uncovered lines

• packages/core/src/platform.ts: L130

[greptile-apps]

Greptile Summary

This PR fixes worktree operations failing with a raw ENOENT in GUI-launched environments (e.g. Windows apps that launch Node without full PATH inheritance) by resolving the Git executable through a proper platform-aware lookup before spawning any git process.

• getGitExecutable() added to platform.ts: walks PATH first using the now-cross-platform findOnPath, then probes a ranked list of well-known install locations per OS (Windows Program Files variants, macOS /usr/bin/Homebrew//usr/local, Linux /usr/bin//usr/local), and caches the result to avoid repeated filesystem probes. Falls back to bare \"git\" only when all candidates miss.
• git() helper in the worktree plugin now catches ENOENT and re-throws with a human-readable message naming the executable that was tried, replacing the raw system error that previously surfaced.
• Tests cover all four branches (PATH hit, Windows fallback, macOS Homebrew fallback, bare-git fallback) using per-test existsSync mocks via vi.doMock + vi.resetModules() + dynamic import() to avoid cross-test cache bleed.

Confidence Score: 5/5

Safe to merge — the change is additive, well-tested across all platform branches, and correctly follows the established caching and cross-platform patterns already in platform.ts.

All changed code paths are covered by explicit tests (PATH hit, Windows/macOS/Linux fallback candidates, bare-git fallback). The findOnPath generalisation preserves existing Windows shell-resolution behaviour because pathDelimiterForCurrentPlatform() still returns ';' on Windows. Caching mirrors the getShell() pattern exactly, and the ENOENT enrichment in git() is a strict improvement with no regression risk.

No files require special attention.

Important Files Changed

Filename	Overview
packages/core/src/platform.ts	Adds cross-platform getGitExecutable() with PATH-walk, platform-specific fallback candidates, and caching; findOnPath is properly generalised from Windows-only to cross-platform using platform-aware delimiters and separators.
packages/core/src/index.ts	Exports getGitExecutable from the core package; _resetGitExecutableCache is correctly kept internal (not re-exported).
packages/plugins/workspace-worktree/src/index.ts	Replaces bare "git" with getGitExecutable() in both git() and exists(); git() now wraps ENOENT with a user-readable message. exists() still silently returns false on all errors, which is the correct semantic for an existence check.
packages/core/src/tests/platform.test.ts	Adds synchronous PATH-walk test and three async branch tests (Windows fallback, macOS Homebrew, Linux bare-git) using vi.doMock + vi.resetModules() + dynamic import() for per-test existsSync isolation; cache reset is properly handled via _resetGitExecutableCache() in beforeEach/afterEach.
packages/plugins/workspace-worktree/src/tests/index.test.ts	Adds getGitExecutable to the @aoagents/ao-core mock so existing worktree tests continue to compile and pass after the new import.

_{Reviews (3): Last reviewed commit: "fix(worktree): satisfy platform test lin..." | Re-trigger Greptile}