docs: untrack internal design notes

[planadecu]

Internal design/planning notes are working documents for the agent tooling, not user-facing
documentation. This removes the last one from version control (it came back in when #18 merged)
and guards against it happening again.

Why

Versioned planning docs go stale and end up contradicting src/. Concretely, the retired x402
plan specified an X402_TESTNET_API_URL constant and a hardcoded testnet URL that were
deliberately removed from the implementation — see the comment in src/types/config.ts.
Nothing in the repo references docs/.

Change

• git rm docs/superpowers/specs/2026-09-21-esm-entry-rework-design.md — the only file under
  docs/, so the folder is now gone.
• .gitignore: ignore /docs so it cannot be silently re-tracked. Optional and trivial to
  drop if you'd rather keep the ability to version docs later.

What is preserved

• The note itself, byte-identical, at
  .claude/docs/superpowers/specs/2026-09-21-esm-entry-rework-design.md (.claude is gitignored).
• The durable rationale for the ESM rework is unaffected: CHANGELOG.md (0.10.0) and
  CLAUDE.md → Build Targets.

Review notes

• No version bump or CHANGELOG entry, deliberately — docs-only, no API or runtime change.
  This deviates from the "bump version before every commit" rule in CLAUDE.md; flagging it so
  the omission is an explicit choice rather than an oversight.
• Merging this does not need a publish, but the workflow publishes a patch on every push to
  main, so merging will cut one regardless.
• pnpm test (53 passing) and pnpm run build ran via the pre-commit hook.