fix(agent): route agent start through the daemon with round-trip verification

[codeaholicguy]

Closes #402.

Confirmed root cause

Split-brain identity stores: agent start writes agents.db under the starting process's $HOME while devkitd (socket + data dir) resolves under its own HOME. Under a nested/sandboxed start (HOME=/tmp/fakehome, overlay envs), the write lands where no client looks — the sweep synthesizes <cwd>-<pid> (slug-lib-75800), send --id can't resolve the name, and start reports a false success. Harness-agnostic; plausibly shares a root with #400.

Fix (scoped per review)

• Core — agent.start RPC in devkitd (crates/devkitd/src/agent_start.rs): the daemon validates (name regex, name-in-use against agents.db + kill(pid,0) liveness, cwd exists), creates the tmux session, polls the pane tree for the deepest harness pid (ported findAgentPid/pollForPid semantics), writes the registry row under its own store (schema-v1 columns so TS migrations take a fresh db forward), and returns the entry. Because the write happens inside the daemon's sane view, both HOME-divergence and write-overlay flavors are covered.
• CLI agent start is a thin client on the tmux provider path: calls agent.start when the socket answers; falls back to local startAgent when the daemon is absent or predates the method. Daemon-side errors surface as command failures.
• Round-trip verification + loud failure (both paths): the local fallback now refuses loudly with AgentHomeDivergenceError before creating any session when $HOME diverges and the account .ai-devkit store exists, warns when divergent-but-self-consistent, and re-reads the registry write before reporting success — a failed round-trip is an error, not a false OK. The daemon path's registry write is authoritative by construction; its errors propagate.
• Autostart guard: ensureDaemon never auto-spawns a second daemon from a divergent HOME ($HOME vs passwd home) — it refuses with a loud diagnostic instead of spawning an invisible twin.
• unregistered:<folder>-<pid>: synthesized identities in the listAgents merge are marked self-explaining instead of masquerading as assigned names.
• Defenses kept: unique type:sessionId → type:cwd re-home on pid drift (generic, all harnesses); dropDescendantProcesses folds spawned child procs into the parent session (TS + devkitd); opencode session.title read tolerantly, surfaced as summary/firstUserMessage metadata only.

Deferred to phase 2 per review: HOME-independent socket path chain ($XDG_RUNTIME_DIR → /tmp → $HOME). Socket stays $HOME/.ai-devkit/daemon.sock as today.

Evidence

• agent_start.rs: name-validation parity with AGENT_NAME_REGEX.
• ManagedAgentRuntime.test.ts: refuses under divergent HOME when the account store exists (no session created), warns+proceeds when self-consistent, fails when the registry write doesn't round-trip — the fake-HOME repro shape, at unit level: via-daemon succeeds with the name or fails loudly, never folder-PID fallback.
• AgentManager.test.ts: registry name survives detected-pid drift, sessionId re-home, ambiguity/no-steal guards, unregistered: expectations, resolveAgent resolves the adopted name.
• fixtures/harness/opencode/spawned.json parity bundle (TS + rust): spawned child folds into parent; slug-lib-75800 title stays metadata.

Test plan

• Scoped: agent-manager 140, daemon-client 14, cargo test -p devkitd 7 — green
• npm run lint, npm test (8 projects), npm run test:e2e (42) — green on hook runs (unrelated ClaudePrintAgentService cold-import flake noted)

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