feat: rewrite spawnpoint in Go, ship as a single binary

[mihirgupta0900]

Summary

Rewrites Spawnpoint in Go so it ships as one native binary instead of needing Python and pipx, and reworks the README messaging and overview video.

• Go port: a feature-for-feature rewrite built on cobra and fang, huh prompts, lipgloss styling, and a custom Bubble Tea fuzzy repo picker.
  • --json and --no-input output match the Python version, which I checked against real workspaces.
  • Config, template and version-cache files stay in the same format, so nothing needs migrating.
• Performance: startup is about 20 ms, down from about 90 ms. sp list on 40 workspaces / 133 worktrees takes 446 ms, down from 913 ms, because the scan now runs in parallel.
• Install: Homebrew cask, curl … | sh (with checksum verification), go install, and PyPI binary wheels, so pipx install / uv tool install keep working.
• sp update: detects how it was installed and runs the matching upgrade (brew, pipx, uv, pip or go). Otherwise it replaces its own binary in place after checking the checksum. It also prints a background notice when a new release is out, which is skipped in --json / --no-input mode.
• Upgrading from the Python version: existing pipx users can run pipx upgrade spawnpoint or the old sp update and they get the Go binary. I verified this end to end with locally built wheels.
• Bug fixes carried over from the Python version:
  • progress bars and wrapped paths no longer leak into --json stdout;
  • add now uses --no-track;
  • single-repo workspaces get the right name;
  • scan_depth is no longer capped at 2.
• Release: GoReleaser v2 plus a wheel builder, still using publish.yml so PyPI trusted publishing keeps working. CI runs the unit tests and the e2e suite.
• Docs and video:
  • the README now leads with multi-repo workspaces for agents, and has install, update, "coming from Python" and uninstall sections;
  • a new 30-second overview video (Remotion) replays the real sp create flow;
  • the video's music and sound effects are synthesized in video/scripts/make_audio.py, so there are no licensing issues.
• The Python implementation (src/, tests/, pyproject.toml, uv.lock) is removed.

Before releasing

• Create the mihirgupta0900/homebrew-tap repo.
• Add a write deploy key to the tap and store its private key as the HOMEBREW_TAP_DEPLOY_KEY secret on this repo.
• Release as a major version bump (1.0.0).
• Optional: upload assets/spawnpoint.mp4 as a GitHub attachment so the README can play it inline with sound. The GIF is silent.

Test plan

• go vet ./... and go test ./... pass, including the e2e suite covering repos, create, add, list/cd, restructure, templates, file compatibility, cleanup, light-cleanup and headless first run.
• Upgrade paths tested locally: the old Python sp update → Go, pipx upgrade, uv tool, pip venv, install.sh, self-update 0.9.0 → 1.0.0, and the install.sh pipx guard.
• Smoke-tested in Alpine and Debian containers.
• Not tested yet: a real Homebrew cask install, running on Windows, and a real GitHub/PyPI release.

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