release: 0.21.0 — MCP CLI surfaces aligned with design doc

[esengine]

Release commit for 0.21.0. Bumps package.json and adds the corresponding CHANGELOG.md entry. No code-path changes here — those landed in #106, #107, #108, #109.

Highlights

MCP CLI surfaces realigned with docs/design/agent-tui-terminal.html sections 24, 32, and 37.

• Lifecycle vocabulary matches design §37: ↻ handshake… / ✓ connected / ✖ failed / ○ disabled (feat(mcp): lifecycle line cards match design doc §37 (Stage A of #105) #106)
• Interactive /mcp browser modal per §24 — keyboard-driven row list with health badges + capability lists, /mcp text retained for non-TTY / replay (feat(mcp): /mcp opens an interactive browser modal (Stage B of #105) #107)
• /mcp disable <name> / /mcp enable <name> persist a mcpDisabled set to config; skipped at next launch with the ○ disabled lifecycle line (feat(mcp): /mcp disable / enable + disabled lifecycle (Stage C1 of #105) #108)
• Per-server p95 slow toast — ⚠ MCP \` slow · 8.4s p95 over the last 5 calls`, idempotent, threshold configurable (feat(mcp): p95 latency tracker + slow-server warn toast (Stage C2a of #105) #109)

Deferred

/mcp reconnect <name> (live tool-list teardown) split out as RFC #110 — the naïve implementation breaks byte-stable prefix when the reconnected tool surface drifts; needs a design call before code. The r keybind in the /mcp browser is a labelled stub.

After merge

• Tag v0.21.0 on the squash-merge commit
• Push tag
• npm publish (after explicit confirmation — public/irreversible)