ProjectStateStore.write_project always fails: it calls DotProjectWriter.write(), which has never existed #574

FilesystemProjectStateStore.write_project (packages/darnit/src/darnit/stores/defaults/project.py:61-71) calls DotProjectWriter(...).write(config). DotProjectWriter has never had a write() method, only update(dict); no commit in dot_project.py's history adds one. So write_project, and write_maintainers, which calls it at line 86, always raise StoreOperationError (wrapping an AttributeError).

It has been broken since the store was added in feature 033 (f24a554, 2026-08-26). Nobody noticed because:

• No production code calls either method. The audit only reads from the project store (tools/audit.py:892), and real .project/ writes go through darnit.config.context_writes (feature 042, the single writer).
• The only test that calls write_project (tests/darnit/stores/test_us1_equivalence.py) uses InMemoryProjectStateStore from darnit-testchecks. The filesystem implementation has never run.
• CI's type check runs mypy on two files with || true. On project.py alone, mypy reports "DotProjectWriter" has no attribute "write" [attr-defined].

Users aren't affected today. But ProjectStateStore.write_project / write_maintainers are part of a plugin-facing Protocol (darnit/stores/protocols.py), so a third-party caller or backend author would build on a method that cannot work.

Proposal: remove write_project and write_maintainers from the ProjectStateStore Protocol and its implementations (filesystem and in-memory). Since feature 042, .project/project.yaml is never written with context values, and context_writes is the only writer (framework-design.md 7.4-7.11, tests/darnit/context_integrity/test_single_writer.py). A generic "write a whole ProjectConfig" method on the store conflicts with that rule.

The alternative is to route them through context_writes with the same restrictions. That keeps a write surface no caller needs.

Found while reviewing #504. A repo-wide audit for other declared-but-unwired interfaces like this one is under way.

Drafted with Claude Code; reviewed and posted by me.