Consolidate duplicated file-write helpers across modules

[JRS1986]

Context

Near-duplicate file-write helpers exist in four modules:

• writers.py:62-71: _write_json, _write_text
• adapters.py:96-113: _write, _write_json, _collect_write
• knowledge.py:58-71: _write, _collect
• enablement.py: inline path.write_text(...) calls

They all do the same thing — mkdir(parents=True, exist_ok=True) + write_text/write_json — with subtly different overwrite semantics (always overwrite vs skip-if-exists vs append-to-skipped-list). Easy place for bugs to creep in as the surface grows.

Recommendation

Create src/coding_scaffold/io.py with three primitives:

def write_text(path: Path, content: str, *, overwrite: bool = True) -> Path: ...
def write_json(path: Path, payload: object, *, overwrite: bool = True) -> Path: ...
def write_if_absent(path: Path, content: str) -> Path | None: ...  # returns None when skipped

Replace all four module-local helpers with these. Update call sites in writers.py, adapters.py, knowledge.py, enablement.py.

Verification

• git grep -nE '_write_json|_write_text|_collect_write|_collect\\b' src/ shows no duplicates.
• All 51 tests still pass.
• Adding a new template-writing call site requires importing one of the three helpers, not copying yet another local function.

Carry-over from prior code review (F8).