v0.4.2 Key/Install Hardening

[t3chn]

Scope

• Add mycel init --file or equivalent explicit file-backend init.
• Add isolated config/data env overrides for smoke tests.
• Add CI-safe local self-send smoke without global keychain dependency.
• Add doctor --json.
• Add release asset workflow and npm install smoke.

Context

v0.4.1 merged the agent contract hardening work, but release smoke still depends on local secret availability and external relay/runtime checks. Keep this follow-up focused on key/install/release hygiene, not A2A/MCP/new adapter work.

Release safety

Before publishing npm packages, GitHub release assets must exist and be reachable because the npm installer downloads binaries from the matching GitHub release tag.