fix(#4460): trim execute-plan.md under its size budget (unrelated regression)

gsd-test surfaced a SEPARATE, unrelated failure while re-verifying this
branch: tests/workflow-size-budget.test.cjs found execute-plan.md at
40981 bytes, 21 over the 40960 DEFAULT hard cap. Root-caused (not
assumed): already-merged PR #4540 (enhance(#4139), unrelated to
#4460/#4459/#4461) added two near-identical explanatory parentheticals
about .compact.md template variants across two nearby steps
(user_setup, create_summary), pushing the file over. Confirmed directly
against origin/next independent of any merge with this branch --
`next` itself already carries this.

This branch's fork point predated PR #4540's merge, so gsd-test's
merge-testing against the current next only now surfaced it (merged
origin/next into this branch in a separate commit first -- 0 conflicts,
after discovering and fixing that this worktree's git clone was
SHALLOW, via `git fetch --unshallow`, which is what made a plain `git
merge origin/next` fail with "refusing to merge unrelated histories").

Fixed by trimming the SECOND (of two near-identical) parentheticals in
the create_summary step to a short back-reference to the first -- same
information, no duplication, no cap raised (the test explicitly warns
against raising it). 40931 bytes, 29 under the cap.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

gsd-core/workflows/execute-plan.md

@@ -409,7 +409,7 @@ emit narrative output between the Write tool call and the commit tool call.
 Truncation at this boundary is a known failure mode (see #2070 rescue logic in
 execute-phase.md step 5.5).
 
-Create `{phase}-{plan}-SUMMARY.md` at `.planning/phases/XX-name/`. Use the template at `~/.claude/gsd-core/templates/summary.md` (or its `~/.claude/gsd-core/templates/summary.compact.md` variant — resolve per `~/.claude/gsd-core/references/compact-content-gate.md` §"Streams 1b and 4").
+Create `{phase}-{plan}-SUMMARY.md` at `.planning/phases/XX-name/`. Use the template at `~/.claude/gsd-core/templates/summary.md` (or its `.compact.md` variant — same `compact-content-gate.md` resolution as the USER-SETUP template above).
 
 **Frontmatter:** phase, plan, subsystem, tags | requires/provides/affects | tech-stack.added/patterns | key-files.created/modified | key-decisions | requirements-completed (**MUST** copy `requirements` array from PLAN.md frontmatter verbatim) | duration ($DURATION), completed ($PLAN_END_TIME date).