feat: named harnesses and interactive template picker

[stephendolan]

Why

Scry is architected as a directory manager that incidentally launches an agent, but in practice it's a conversation launcher where the directory is incidental. This makes the two things that actually vary per launch — the harness and the template — first-class and selectable at creation time.

Harnesses (--harness)

Previously the launched command was a single global agent string with no per-invocation override. Now:

• Config gains a harnesses name→command map and default_harness.
• --harness NAME (or SCRY_HARNESS) selects a configured harness; an unknown name fails fast before the TUI opens.
• The legacy agent string is still honored when no harnesses map is configured, so existing configs keep working (dual-read).

{
  "default_harness": "claude",
  "harnesses": {
    "claude": "claude --dangerously-skip-permissions",
    "codex": "codex"
  }
}

Template picker

Creating a new scry previously silently auto-applied the default template. That's a poor default when default is a heavyweight, specific context. Now:

• New scries open an interactive template picker; if a default template exists it's preselected, so Enter still gives the old behavior — but it's visible and overridable.
• --template NAME still skips the picker.
• Hidden dirs are excluded from the template list (a stray .claude dir no longer shows as an option).

Notes

• Kept the Crystal stack — single static binary + instant startup is exactly what a shell-integrated cd-wrapper needs; a rewrite would buy nothing observable.
• Version bumped to 0.9.0 to trigger release.
• crystal spec -Dspec, ameba, and crystal tool format --check all pass; added coverage for resolve_harness and the picker flow.

🤖 Generated with Claude Code