feat(routing): auto-detect dario at startup, drop the manual env-var dance

[askalf]

Summary

When `ANTHROPIC_BASE_URL` isn't already set, probe `localhost:3456/health` at the start of `hands run`. If dario responds, set `ANTHROPIC_BASE_URL` so the Anthropic SDK routes through it for OAuth subscription billing. Operator override always wins: env-pre-set or `--no-dario` skip the probe.

Why

Today's E2E verification (dario #172 + dario #174) proved hands SDK mode + dario gives OAuth subscription billing instead of per-token API key cost. Friction was the only barrier left — users had to remember `export ANTHROPIC_BASE_URL=http://localhost:3456\` before every `hands run`. This makes the subscription path automatic when dario is up; silent fall-through to `api.anthropic.com` when it isn't.

Behavior

Condition	Outcome
`ANTHROPIC_BASE_URL` already set	Respect it, no probe
`--no-dario` flag	Skip probe, no env var change
Dario reachable on `localhost:3456/health`	Set env var, log info line
Dario not reachable (timeout / refused / non-OK)	Silent fall-through to `api.anthropic.com`
`HANDS_DARIO_URL` env set	Probe that URL instead of default

Probe target: dario's `/health` endpoint (already used by `hands doctor` — same shape, decoupled by route from any auth-bearing path). 2s timeout — dario's first `/health` on a cold proxy was observed at ~840ms (account pool + template state checks); subsequent hits much faster, but each `hands run` invocation runs the probe once so we budget for cold-path.

Test plan

• Unit: `test/dario-detect.test.mjs` — 7 cases covering operator-override (env pre-set), explicit disable, successful probe, non-OK response, network error fall-through, `HANDS_DARIO_URL` override, trailing-slash trimming.
• Live: ran `hands run` against a running dario without setting `ANTHROPIC_BASE_URL`. Auto-detected, routed through, `five_hour` billing on dario's log.
• Local: `npm test` passes 56/56.
• CI: standard hands checks.

Notes

• Pure additive — three files modified (`run.ts` adds 12 lines, `cli.ts` adds 1 flag), one new file (`dario-detect.ts`), one new test file. No runtime path changes outside the new probe.
• Operator-override semantics preserved across both pre-set env var and explicit `--no-dario` flag — there's no path where the auto-detect can override an operator's intent to use `api.anthropic.com`.