hand spawn fails: treehouse v2 dropped --json from get --lease

[alchemicwebstudio]

Bug

`hand spawn ` fails immediately:

```
acquire treehouse worktree: treehouse get failed: exit status 1: unknown flag: --json
```

Cause

`internal/worktree/worktree.go`'s `Get()` calls:

```go
args := []string{"get", "--lease", "--json"}
```

and parses stdout as JSON (`{"path": "..."}`).

The locally installed treehouse CLI is v2.0.0. Its `get --lease` no longer
accepts `--json` at all (not in `treehouse get --help`'s flag list) and
instead prints the plain worktree path directly to stdout per its own
`--lease` help text:

Durably lease a worktree without opening a subshell; print only its path
to stdout

So `hand` is targeting an older treehouse contract that v2.0.0 no longer
implements. Every `hand spawn` is broken against current treehouse until
this is reconciled (either hand parses the plain-path output, or treehouse
pins/restores `--json`).

Repro

```
treehouse --version # v2.0.0
hand spawn rhb-product-page rockhardbeds

-> acquire treehouse worktree: treehouse get failed: exit status 1: unknown flag: --json

```

Suggested fix

Update `internal/worktree/worktree.go` `Get()` to read the plain path from
stdout (trim whitespace) instead of JSON-unmarshaling, matching treehouse
v2's documented `--lease` contract. Worth checking `internal/worktree/worktree_test.go`
and `cmd/spawn_test.go`'s fake treehouse script for the same JSON assumption.

[atqamz]

Not reproducible against current treehouse. The flag is back, and hand is unchanged.

Verified on this host today:

$ treehouse --version
v2.1.0

$ treehouse get --help
      --json    Print lease allocation as JSON (requires --lease)
      --lease   Durably lease a worktree without opening a subshell; print only its path to stdout

and the help text now reads "By default it prints only the absolute path to stdout; add --json for the lease identity and metadata."

End-to-end proof rather than a help-text reading: hand spawn universe-th-prune universe succeeded against treehouse v2.1.0 and returned a real leased worktree at /home/atqa/.treehouse/universe-a49681/1/universe. internal/worktree/worktree.go still passes --json and still unmarshals {"path": ...}, unmodified.

I could not verify the v2.0.0 side of the report directly, since that version is not installed here. Taking the report at face value, the timeline is:

• v2.0.0 (2026-06-24) drops --json, breaking every hand spawn. This issue is filed.
• v2.1.0 (2026-07-20) ships "pool: add stable lease identities" (feat(pool): add stable lease identities kunchenguid/treehouse#68), which reintroduces --json on get --lease as the way to surface the new lease identity and metadata.

The nuance that matters: --json did not come back as a compatibility fix. It came back incidentally, as the surface of an unrelated feature. Nothing upstream is holding that contract still for us, and no version floor is recorded anywhere in this repo, so a fresh install on v2.0.0 or v2.0.1 still fails with unknown flag: --json and no hint as to why.

Narrowing this issue

The code fix originally suggested here is no longer the right change; parsing plain-path output would now be writing a fallback for a superseded version. What is left is the documentation gap:

• State a minimum supported treehouse version (>= v2.1.0) in the README alongside the existing treehouse dependency note.

Separate follow-up, not in scope here

Now that treehouse exposes stable lease identities, internal/worktree/worktree.go CheckCollision may no longer need to hand-roll a guard by comparing worktree path strings against every active task, which is the workaround for the stale-lease-after-crash bug (firstmate #947). Worth evaluating against the new lease identity on its own issue.