Fix: gstack-config get returns "" with exit 0 for keys missing from the DEFAULTS table

[benjaminberes-bp]

Skill preambles read configuration with

VAR=$(gstack-config get <key> 2>/dev/null || echo "<default>")

That fallback only fires on a non-zero exit. lookup_default ended in a
catch-all that echoed "" and returned 0, so for any key absent from the table
VAR came back empty and the default written right there in the preamble was
unreachable. The skill then branched on a value it never specified — "skip
entirely if QUESTION_TUNING: false", reached with QUESTION_TUNING="".

Keys affected

Four keys that skills actually read had no entry:

key	value the call sites already assume
question_tuning	false
repo_mode	unknown
team_mode	false
transcript_ingest_mode	off

Each default added here is taken from the call sites own || echo fallback, so
this only makes reachable what was already intended.

Scope of the exit-code change

The catch-all now returns non-zero, and that is deliberately limited to the
unknown-key arm. Keys whose default is intentionally empty keep exit 0,
because "" is their real answer and their callers depend on receiving it:

• cross_project_learnings — # intentionally empty, unset triggers first-time prompt
• redact_repo_visibility — # empty, falls through to gh/glab detection
• salience_allowlist, user_slug_at_*

Making every empty answer an error would have broken those, which is why the
change is not "exit non-zero when the value is empty".

Test

test/gstack-config-defaults.test.ts pins the class rather than the four
instances. It parses the case arms and asserts that every
gstack-config get <key> site in the tree is covered by one, so adding a read
without adding its default fails CI. It also pins the exit-code contract in
both directions.

Verified failing against the pre-fix script, where it names exactly those four
keys, and passing after.

Found on Windows 11 / git-bash / bun 1.3.14, but nothing here is
platform-specific.

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