feat(personas): named system-prompt overrides for hands run

[askalf]

Summary

Adds two new flags to `hands run`:

```
--persona use a named persona (bundled or ~/.hands/personas/.md)
--system-prompt use an arbitrary prompt file (bypasses --persona)
```

The resolved prompt replaces hands' default OS-aware system prompt in SDK mode. Default behavior (no flag) is unchanged.

Bundled personas:

Name	Behavior
`minimal`	Short, no constraints, "use tools when useful"
`thorough`	Take initiative, exhaustive code with comments
`concise`	Terse, one-or-two sentences, no preamble
`security-aware`	Confirm-before-destructive; read-only OK without

Resolution order for `--persona`:

1. `~/.hands/personas/.md` (user override; takes precedence)
2. One of the four bundled personas above
3. Error: `persona '' not found` with the bundled set listed

`--system-prompt ` bypasses persona lookup entirely — direct file load.

Why this is safe to ship

dario research #172 confirmed that Anthropic's billing classifier doesn't fingerprint system prompt content — content, length, and block count are not classifier inputs as long as the rest of the request shape (effort, max_tokens, tool array, body field order, billing tag, anthropic-beta) is preserved. Combined with hands routing through dario for OAuth subscription billing, that means swapping the system prompt does NOT flip billing from `five_hour` to `overage`. Personas are the operator-facing surface for that capability.

Mutex + early-exit

`--persona` and `--system-prompt` are mutually exclusive — both set exits 1 with a clear error before doing anything else. Resolution happens BEFORE config load and screenshot capture, so "persona not found" surfaces fast with a helpful error message that lists the bundled set.

Scope: SDK mode only (for now)

Plumbed through `runSdkMode` (the API-key / dario-routed path). CLI mode (spawning `claude --append-system-prompt`) doesn't plumb `--persona` through yet — the integration there is meaningfully different (`--append` vs replace) and deserves its own PR. Today's verification is SDK-mode + dario.

Test plan

• Unit: `test/personas.test.mjs` — 7 cases covering the bundled set, unknown name (helpful error message), user-file override beats bundled, content trimming on user-file path, explicit `--system-prompt` path loading, missing file throws, `listBundledNames` identity. HOME-redirect via tmpdir for user-file tests.
• Local: `npm test` passes 56/56.
• CI: standard hands checks.

Notes

• New file: `src/personas.ts`. Three modified: `run.ts` (resolution + plumbing), `sdk-mode.ts` (accepts `systemPromptOverride`), `cli.ts` (flags). Pure additive on every existing path; default behavior unchanged.
• This is the second of three features pending today (auto-detect dario was #26). Replay mode is the third — separate PR.