docs: docstring policy pass over modern_di

[lesnik512]

Applies the docstring policy from the 2026-07-06 verbosity benchmark across modern_di/: public API documents the contract; internal helpers get a one-line contract plus at most 1–2 lines for a genuinely non-obvious constraint; implementation narration and reviewer-justification deleted. The policy is added to CLAUDE.md's Code Style section. Spec: planning/changes/2026-07-07.02-docstring-cleanup.md.

Highlights:

• wiring.py: 42 docstring lines in a 159-line file trimmed; the stale _compile_kwargs / Factory._find_dep_provider references (functions removed long ago) are gone; the fresh-ArgumentResolutionError rationale is stated once instead of three times.
• container.py: resolve_provider keeps its contract, loses the RecursionError-conversion narration (lives in architecture/validation.md); _convert_recursion_error, _warn_and_reopen_if_closed, __init__, set_context trimmed to contract + load-bearing constraints.
• exceptions.py: DependencyPathMixin keeps the slots constraint, loses the reviewer-justification; the # message built dynamically; not templated comment (3×) deleted; per-category filterwarnings recipes on warning classes kept (public contract).

Prose-only diff — no code, message-string, or test changes (review verified stripped ASTs are identical for every changed file), and every deleted rationale was verified to survive in architecture/ or on its owning class.

Gates: just test-ci green (277 passed, 100% coverage), just lint-ci green.