tool-build: re-scope + build MyCoder (issue → diff → PR)

[lorenzoliuzzo]

Part of the fleet milestone (#1 §2). Highest-leverage remaining tool.

MyCoder (issue → diff → PR) is the only designed tool that writes the actual
feature/fix. It was deferred pending a real Engine backend — that blocker is
gone: ClaudeCLIEngine shipped in mythings-core (2026-07-07).

This issue = re-scope + build

• Re-scope MyCoder against the shipped ClaudeCLIEngine and the current
  contracts (isolation, github.open_pr, ledger).
• Define the safety envelope up front: read-only allowlist, one-repo
  worktree isolation, draft-PR-only, never self-merge — and how it composes
  with the fleet's spend cap (safety: enforce a hard spend/turn cap before headless sessions #8) and (eventually) the GitHub App (safety: introduce a permission-scoped GitHub App identity (replace personal PAT) #7).
• Scaffold + implement a minimal end-to-end (tiny issue → diff → draft PR)
  + tests.

Note the coupling: MyCoder makes several other gaps urgent (#7 GitHub App,
#8 spend cap, #9 secret-leak). Sequence accordingly.