Drop the hard jq dependency from gstack-gbrain-source-wireup

[benjaminberes-bp]

check_source_state() shells out to jq and hard-fails without it:

gstack-gbrain-source-wireup: jq required for source state detection. Install jq (brew install jq) and re-run.
WARNING: gbrain wireup failed; run .../gstack-gbrain-source-wireup manually after fixing prereqs

Two separate problems.

The dependency is unnecessary

The comment justifying it is stale:

# jq is documented as a dependency of gstack-gbrain-detect (transitive)

gstack-gbrain-detect is TypeScript and no longer shells out to jq — its own
comments refer to "the bash version's jq -n" in the past tense. So jq had
become a hard dependency of this one code path, for exactly two identical
extractions:

jq -r --arg id "$id" '.sources[] | select(.id==$id) | .local_path'

bun is already a hard dependency of gstack — several bins carry a
#!/usr/bin/env bun shebang. Parsing the JSON with it removes a dependency
rather than documenting one.

The advice is not actionable

brew install jq does not apply on Windows/git-bash, where jq is absent by
default. The practical effect there is that the gbrain wireup is skipped on
every run, as a warning the run continues past. brain-restore carrying on
is the right call, but the breakage becomes routine and stops being read.

What is preserved

The loud-failure property adversarial review asked for. Without a JSON parser
we still cannot distinguish absent from missing-tool, and would falsely
re-add an existing source — so check_source_state still probes and still
dies. Only the tool it probes for changed, to one that is already required.

Verification

On Windows 11 / git-bash / bun 1.3.14:

• --probe returns byte-identical output with jq present and with jq
  removed from PATH, where it previously died.
• The bun extraction was checked against jq on real
  gbrain sources list --json output, plus the absent-id and invalid-JSON
  cases, which both yield "" as before.

Happy to keep a jq fast path if you would rather not add a bun call in a bash
bin — but given bun is already required, this seemed like the smaller surface.

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