feat(init): prompt for harness when --ai is absent (TTY only) (#36)

[kevinkod]

Summary

• specflow init no longer silently defaults to Claude when the user hasn't passed --ai. In an interactive terminal it now lists the 8 supported harnesses (Claude Code, Cursor, Codex, Gemini, Windsurf, Copilot, OpenCode, Antigravity) and asks the user to pick one — Enter selects the Claude default, invalid input re-prompts.
• When stdin is not a TTY (CI, scripts, piped input), the silent Claude default is preserved — zero regression for automated usage.
• Multi-select / multi-harness in a single project remains out of scope (see Prompt for harness selection on init when --ai is absent and stdin is a TTY #36 for the rationale on lock-format and inter-harness conflict resolution).

Implementation notes

• New pure module src/cli/harness_picker.ts encapsulates the list, default, and re-prompt loop, with an injectable readLine callback so the picker logic is fully unit-testable without a real TTY.
• Parser: intent.ai becomes Harness | null. The handler resolves null to a concrete harness via Deno.stdin.isTerminal() + pickHarness().
• TerminalPrompt is left untouched — its select() doesn't support "empty-input as default" and the picker has its own loop semantics, so reusing it would have required a richer signature than this ticket needs.

Test plan

• Unit tests cover every branch of the picker: empty → default, each 1-based index → matching harness, out-of-range / non-numeric → re-prompt, whitespace-only → default, null (EOF/Ctrl-D) → default.
• Existing integration test renamed to specflow init (no --ai) defaults to Claude in non-TTY environments to reflect what it actually exercises (Deno.Command pipes stdin, so isTerminal() is false).
• Sandbox E2E (non-TTY): specflow init --here from working tree against a fresh Vite project → no prompt shown, lock written with harness: claude, "Open the project in Claude Code" banner appears.
• Local: deno task test — 339 passed.
• CI green on macOS / Ubuntu / Windows cross-smoke.

Closes #36

🤖 Generated with Claude Code