Delete docs/v0.3.0-design.md and strip all references to it

The design doc and its followups doc are no longer needed as a live
reference now that the v0.3.0 redesign is implemented — comments and
docstrings across the codebase cited it extensively (file path, "design
doc §X.Y", "decision N", or bare "§X.Y" section numbers) as design
rationale. Removed docs/ and edited every citing comment/docstring to
drop the now-dangling reference while keeping the substantive
explanation next to it. CLAUDE.md's v0.3.0 roadmap bullet loses its
trailing pointer to the deleted file.

Verified: no remaining "docs/v0.3.0", "design doc", "decision N", or
"§N.N" references (repo-wide grep); ruff and ty clean; full test suite
on the heaviest-touched modules (network, sample, rollout, migration,
config, train) passes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_cli_train_overrides.py

@@ -1,6 +1,5 @@
-"""Tests for `giant train`'s stage-prefixed CLI flags (docs/v0.3.0-design.md
-decision 7 / docs/v0.3.0-followups.md item 2): --stage1-*/--stage2-* must
-independently override each stage's config block, and must take precedence
+"""Tests for `giant train`'s stage-prefixed CLI flags: --stage1-*/--stage2-*
+must independently override each stage's config block, and must take precedence
 over the older shared flags (--mode/--hidden-dim/--n-critic/... ) that still
 apply the same value to both stages for backward compatibility."""