(MOT-3873) feat(harness): code mode with native coder/shell exposure

[ytallo]

• new send/spawn mode "code": replaces the mesh identity prompt with a
  dedicated coding identity (prompts/code.txt) — read-before-edit,
  str_replace edit discipline, verify-via-tests, no-commit-unless-asked.
• code mode without an explicit functions policy defaults to the curated
  coding surface natively exposed (coder::*, shell::exec/exec_bg/kill/
  status); an explicit caller policy always wins, and spawned children
  still narrow through subset_policy.
• code-mode turns fetch coder::context once per turn (jail-scoped via the
  trusted fs_scope stamp) and append a rendered block (root,
  platform, git state, AGENTS.md/CLAUDE.md) to the system prompt; an
  unavailable coder surface logs and proceeds without the block.
• send/spawn wire schemas gain the "code" mode enum value (goldens).

Refs MOT-3873

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skill-check — worker

0 verified, 32 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.

[ytallo]

Consolidated into #423.