feat: hopx shell — create a sandbox and open a terminal in one step

[alexo-bunnyshell]

Summary

Connecting to a fresh sandbox took two commands — hopx sandbox create, then copying the printed ID into hopx terminal <id>. This PR adds hopx shell (alias sh) that does both in one step.

hopx shell                      # default template, terminal opens
hopx shell -t code-interpreter
hopx shell -e KEY=value --timeout 600
hopx shell --rm                 # ephemeral: kill the sandbox on exit

• Creates the sandbox and attaches an interactive terminal directly to the live object returned by Sandbox.create() — no reconnect round-trip.
• Reuses create's provisioning options: --template/-t, --timeout, --env/-e.
• Prints the sandbox ID on connect, so even an ephemeral session is recoverable until exit.

Lifecycle on exit

• Default: the sandbox keeps running after you exit; reattach with hopx terminal <id>.
• --rm: ephemeral — the sandbox is killed when the session ends (à la docker run --rm).

Refactor

The interactive-terminal plumbing (raw stdin forwarding, resize handling, cleanup) was extracted from terminal.ts into src/lib/terminal/session.ts and is now shared by both terminal and shell. No behavior change for hopx terminal (it's now a thin wrapper). The shared helper takes an onExit hook, which --rm uses to kill the sandbox before exit.

Changes

• src/commands/shell.ts — new command
• src/lib/terminal/session.ts — shared attachInteractiveTerminal() helper
• src/commands/terminal.ts — refactored onto the helper (~70 fewer lines)
• src/index.ts — register shellCommand
• Docs: README (commands table, quick-start, "Interactive Shell" section), CLAUDE.md, CHANGELOG
• Version bump 0.2.2 → 0.3.0 (package.json + version.ts)
• Unit test for the shell command shape
• chore commit: pin bun via mise.toml, gitignore mise.local.toml (local secrets)

Testing

• bun test (offline): 200 pass, 0 fail
• E2E against prod (SKIP_E2E=0, real key): 21 pass, 0 fail
• hopx shell --help / --version verified; tsc --noEmit clean for all touched files

Note: a full end-to-end manual hopx shell session against a live backend wasn't run in CI; worth a quick manual smoke test before release.

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