feat(em): add agy (Antigravity CLI) as em-ai backend

[Data-Wise]

Summary

• Adds agy (Antigravity CLI) as the primary fallback AI backend in em's AI abstraction layer, ahead of gemini in the chain (claude → agy → gemini → none). Google's gemini CLI is deprecated upstream in favor of Antigravity; gemini is kept, not removed, for existing FLOW_EMAIL_AI=gemini configs.
• agy needs a validation wrapper, not a drop-in swap: it has no clean-output flag (appends an unrequested markdown status block to every response) and has been observed to exit 0 with a canned "system operational" response on degenerate input rather than failing loudly. New _em_ai_agy_clean_output/_em_ai_agy_looks_valid helpers strip and sanity-check its output before use; a failed check falls through to the existing fallback chain.
• Fixes a more serious bug found during live verification: capturing agy's output via $(...) command substitution hangs or dies silently, independent of timeout — agy leaves a descendant process attached to its stdout pipe, so $(...) blocks waiting for an EOF that never comes. Fixed by routing output through a temp file instead of $(...).
• Updates doctor's health check and the full doc surface documenting the --backend/FLOW_EMAIL_AI option set: EMAIL-DISPATCHER-GUIDE.md, EMAIL-COOKBOOK.md, docs/commands/doctor.md, MASTER-DISPATCHER-GUIDE.md, MASTER-API-REFERENCE.md, MASTER-ARCHITECTURE.md, REFCARD-EMAIL-DISPATCHER.md, QUICK-REFERENCE.md, docs/tutorials/46-em-v2-features.md, man/man1/em.1, root + docs CHANGELOG.md. Left docs/specs/* (historical, point-in-time) and docs/tutorials/33-himalaya-email.md's himalaya.nvim :HimalayaAi setting (a separate editor plugin, not em) untouched.
• Registered tests/test-em-ai-switch.zsh in run-all.sh, which had never been wired into the suite.

Test plan

• tests/test-em-ai-switch.zsh — 24/24 pass (added agy switch/toggle/extra_args cases)
• tests/test-em-ai-agy.zsh (new) — 8/8 pass (clean-output stripping + canned-response validation)
• ./tests/run-all.sh — 77 passed, 0 failed, 0 timeout, 1 skipped (expected — e2e-em-dispatcher skips when the external tool is absent, matches documented baseline). Ran twice for consistency.
• mkdocs build --strict — clean, run twice (before and after the second docs pass), 0 warnings both times.
• Live e2e: _em_ai_execute agy "..." "..." 15 classify verified against the real agy binary post-fix — returns hello/EXIT:0 cleanly through the actual code path (pre-fix: hung/died silently, reproduced 3x).

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