* feat: GSTACK_* env-key shim for Conductor workspaces

New lib/conductor-env-shim.ts promotes GSTACK_ANTHROPIC_API_KEY and
GSTACK_OPENAI_API_KEY to canonical names when canonical is empty. Wired
into the four TS entry points that hit paid APIs or gbrain embeddings:
gstack-gbrain-sync.ts, gstack-model-benchmark, preflight-agent-sdk.ts,
test/helpers/e2e-helpers.ts. Side-effect-only import, 15 lines total.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs: gbrain+gstack setup, Conductor env mapping (v1.39.2.0)

USING_GBRAIN_WITH_GSTACK.md: new "What you get after setup" section,
Path 4 (remote MCP / split-engine), /sync-gbrain workflow stages +
watermark mechanics, "Conductor + GSTACK_* env vars" section, env vars
table extended, two troubleshooting entries (silent embedding failure
and FILE_TOO_LARGE watermark block).

CONTRIBUTING.md "Conductor workspaces": new paragraph on the GSTACK_*
prefix pattern and the four entry points importing the shim.

VERSION 1.39.1.0 → 1.39.2.0 and CHANGELOG entry covering the shim +
docs (full release-summary format with before/after table).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* test: unit coverage for conductor-env-shim

Refactor lib/conductor-env-shim.ts to export promoteConductorEnv()
so unit tests can manipulate env and call it directly (a bare side-
effect IIFE on import isn't reachable from bun:test once cached).
The on-import IIFE still runs — existing four-entry-point imports
keep working unchanged.

test/conductor-env-shim.test.ts covers all three branches:
GSTACK_FOO present + FOO empty → promotion; FOO already set →
no-overwrite; nothing in env → no-op.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs: Conductor strips canonical API keys (not just "doesn't inherit")

The prior docs framed the GSTACK_* prefix as collision-avoidance:
"Conductor exposes API keys under a GSTACK_ prefix so it never
collides with whatever the host system has set." That understates
the mechanism — Conductor actively strips ANTHROPIC_API_KEY and
OPENAI_API_KEY from every workspace's process env, so setting them
in ~/.zshrc or .env doesn't help. The fix path is to set the
GSTACK_-prefixed forms in Conductor's workspace env config; Conductor
passes those through untouched.

Three docs updated to reflect the strip, not the polite framing:
USING_GBRAIN_WITH_GSTACK.md (Conductor section), CONTRIBUTING.md
(Conductor workspaces paragraph), CHANGELOG.md (release summary).

README.md gains a "Running gstack in Conductor?" callout in the
GBrain section pointing at the canonical doc's anchor, plus a fourth
path entry (remote gbrain MCP / split-engine) that was already
documented in USING_GBRAIN but missing from the README summary.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>