Extract inline Markdown/JSON/YAML templates out of writers.py and adapters.py

[JRS1986]

Context

src/coding_scaffold/writers.py is ~864 lines, of which roughly 700 are multi-line string literals. src/coding_scaffold/adapters.py follows the same pattern with embedded TypeScript and Markdown. Real logic is buried under template walls, diffs are noisy, and the templates themselves can't be linted, syntax-highlighted properly, or previewed standalone.

Recommendation

• Move templates into src/coding_scaffold/templates/ as real .md, .json, .yaml, .ts, and .txt files.
• Load via importlib.resources.files("coding_scaffold.templates").joinpath(...).
• Use str.format or a small placeholder helper for variable substitution ({weak_model}, {strong_model}, {routine_threshold}).
• Keep modules focused on orchestration (read inputs, pick template, fill placeholders, write output).

Verification

• writers.py and adapters.py shrink to a few hundred lines each.
• All 51 tests still pass.
• git grep -n '\"\"\"' src/coding_scaffold/writers.py src/coding_scaffold/adapters.py returns far fewer matches.
• Templates are visible/editable without scrolling through Python.

Carry-over from prior code review (F4).