fix(#214): apply OpenCode write-truncation contract to all large-file writer agents (#599)

* fix(#214): apply OpenCode write-truncation contract to all large-file writer agents

Issue #214 / PR #598 fixed gsd-phase-researcher's OpenCode write-tool
truncation by adding a single-Write-default + sentinel-based
Write->Read->Edit incremental fallback contract to its Step 6. The root
cause is upstream opencode#18108: OUTPUT_TOKEN_MAX=32000 is shared with
the thinking budget, so a single oversized `write` tool call's JSON is
truncated mid-payload (`JSON Parse error: Expected '}'`) and OpenCode
doom-loops.

The same failure affects every GSD subagent that writes a large file in
one Write call. Mirror the phase-researcher write contract (adapted per
output filename) into the other large-file writers:

- gsd-research-synthesizer (SUMMARY.md) — extends the existing bug-222
  hard-rules block with the truncation fallback as rule 6, preserving
  every original rule
- gsd-planner (PLAN.md)
- gsd-executor (SUMMARY.md)
- gsd-domain-researcher (AI-SPEC.md Section 1b)
- gsd-project-researcher (.planning/research/*.md)
- gsd-ui-researcher (UI-SPEC.md)

Each keeps the single-Write default (no behavior change for Claude Code
and other non-truncating runtimes) and falls back to incremental,
sentinel-based section-by-section writes only on a truncation/invalid-tool
failure; never silently falls back to returning content.

Locked with a parametrized prompt-contract regression test mirroring the
bug-214 / bug-222 pattern across all six agents.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#214): set changeset pr to 599

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

agents/gsd-executor.md

@@ -589,6 +589,19 @@ After all tasks complete, create `{phase}-{plan}-SUMMARY.md` at `.planning/phase
 
 Use the Write tool to create files — never use `Bash(cat << 'EOF')` or heredoc commands for file creation.
 
+**Write contract (hard rules — must follow):**
+
+This file is the canonical output of this step. The orchestrator reads `.planning/phases/XX-name/{phase}-{plan}-SUMMARY.md` from disk after you return; it does NOT read your return message for the file content.
+
+1. **Default: write the whole file in a single `Write` call.** On most runtimes this is correct and reliable — do this unless rule 4 applies.
+2. **Do NOT return the SUMMARY.md content in your response.** Your return message is a brief confirmation; the content lives on disk.
+3. **Do NOT use `Bash(cat << 'EOF')` or heredoc** for file creation. Use the `Write` tool.
+4. **Large-file / truncation fallback.** Some runtimes (e.g. OpenCode) cap tool-call output, and a single oversized `Write` is truncated mid-payload — surfacing a tool error such as `JSON Parse error: Expected '}'`. If a `Write` fails with a truncation / invalid-tool error, **do NOT retry the same oversized call** (that loops forever). Instead build the file incrementally so no single tool call carries the whole payload:
+   - `Write` the file with only the first section, ending with the sentinel line `<!-- gsd:write-continue -->`.
+   - `Read` the file, then `Edit` it, replacing `<!-- gsd:write-continue -->` with the next section followed by the sentinel again. Repeat, one section per `Edit`.
+   - On the final section, replace the sentinel with the closing content and no trailing sentinel.
+5. **If writing still fails, surface the actual error in your return message.** **Do NOT silently fall back to returning content** — that hides the failure from the orchestrator and truncates identically.
+
 **Use template:** @~/.claude/get-shit-done/templates/summary.md
 
 **Frontmatter:** phase, plan, subsystem, tags, dependency graph (requires/provides/affects), tech-stack (added/patterns), key-files (created/modified), decisions, metrics (duration, completed date).