fix(input-helper): self-heal missing +x on packaged Swift binary (#32)

[mrz1880]

Summary

Closes (partially) #32 — the runtime side of the fix discussed in that issue.

• Distinguish binary missing from binary present but not executable in runInputHelper instead of collapsing both into BINARY_NOT_FOUND.
• Attempt a one-shot chmod 0o755 self-heal when the file exists without +x. This recovers the vast majority of broken installs in the wild today (npx cache, --ignore-scripts consumers, anything laid down before a publish-time fix lands) with zero user action.
• Rewrite the error messages so they point at the real problem: BINARY_NOT_EXECUTABLE includes the actual path so the user knows exactly which file to chmod; BINARY_NOT_FOUND no longer sends npx users to pnpm run build:swift (a dead end without a source tree) and instead splits guidance for npx vs. local-checkout consumers.

This is meant to complement, not replace, the publish-time prepublishOnly mode-assertion proposed in the issue thread — that's still the right primary fix. The two together would protect future releases (prepublishOnly) while rescuing every cached install that already exists on disk today (this PR).

What changed

• src/helpers/input-helper.ts — new ensureBinaryExecutable() helper:
  • access(X_OK) first → fast path, unchanged behavior when everything is fine.
  • On failure, access(F_OK) to disambiguate. Missing → BINARY_NOT_FOUND with path in the message.
  • File exists → chmod(BINARY_PATH, 0o755) then re-check X_OK. Any failure in that sequence → BINARY_NOT_EXECUTABLE with the path and the original error preserved as cause.
• src/constants.ts — added ERROR_MESSAGES.BINARY_NOT_EXECUTABLE; reworded BINARY_NOT_FOUND to be actionable for both install paths.
• src/__tests__/helpers/input-helper.test.ts — added two new tests:
  • self-heals via chmod when binary exists but lacks +x (asserts chmod is called with 0o755 and execution proceeds).
  • throws BINARY_NOT_EXECUTABLE when binary exists but chmod fails (asserts execFile is never invoked).
    Existing test for the missing-file path still covers BINARY_NOT_FOUND via substring match on the message.
• CHANGELOG.md — entry under [Unreleased] linking to dist/bin/input-helper is published without executable bit — every Swift-backed tool fails with misleading "build:swift" error #32.

Test plan

• pnpm test — 184 tests pass (was 181; +3 for the new behavior).
• pnpm lint — clean.
• pnpm build — clean.
• Manually reproduced the original failure on macOS arm64 with npx -y mac-use-mcp@1.1.1, then verified that on this branch the first tool call (screenshot) succeeds without any manual chmod.

Notes for reviewers

• The new catch clause in ensureBinaryExecutable is bare (no error binding) because the X_OK error gets superseded by either the F_OK error (missing file path) or the heal-attempt error (chmod path) — preserving the originally-caught error there would mask the more informative downstream cause. The lint rule preserve-caught-error is happy with this shape.
• BINARY_PATH is appended at the throw site rather than templated into the constant so the constants table stays a plain as const lookup (consistent with how TIMEOUT works).
• No behavior change on the happy path: access(X_OK) returns immediately and we proceed to execFile exactly as before.