Shell Command Projection Module (expanded glossary entry, 2026-05-13)

Module owning all OS-facing I/O for the tool: runtime-aware command-text rendering (hook commands, PATH action lines, shim scripts), subprocess dispatch (run-git, run-npm, run-tool, probeTty), and platform file I/O (platformWriteSync, platformReadSync, platformEnsureDir). Single seam for platform-conditional logic — one place to fix any shell or file write regression across Windows, macOS, and Linux. Lives in get-shit-done/bin/lib/shell-command-projection.cjs. See ADR-0009 (superseded "does not execute" constraint) and ADR-0010 (superseded File Operation Engine).

Invariants:

• Result shape: all run-* return \{ exitCode, stdout, stderr \}; never throw on non-zero exit code.
• Platform policy owned at the seam: shell: process.platform === 'win32' lives only in run-npm; probeTty returns null on Windows.
• Normalization policy: platformWriteSync owns full normalizeMd for .md; CRLF-to-LF + trailing newline for all others; callers must NOT pre-call normalizeMd.
• _normalizeMd is re-implemented inline (not imported from core.cjs) to avoid circular dep.
• atomicWriteFileSync, safeReadFile, normalizeMd remain in core.cjs exports until Phase 4 (#3468).

Migration plan: Phase 1 (#3465) seam additions complete; Phase 2 (#3466) targets 6 subprocess files; Phase 3 (#3467) targets 15 fs files (215 call sites); Phase 4 (#3468) removes compat exports.