fix: select OpenCode build agent for workers

[osantis]

What Changed

• Select OpenCode’s build agent for ship and scout workers while preserving default-agent semantics for secondmates.
• Add dispatch-profile coverage for OpenCode ship, scout, and secondmate launch commands.
• Document the launch behavior and version-scoped OpenCode 1.18.9 verification evidence.

Risk Assessment

✅ Low: Captain, the change is narrowly scoped, preserves secondmate semantics, covers both worker kinds, and aligns documentation with version-scoped evidence.

Testing

The focused spawn suite passed for OpenCode worker, scout, and secondmate launch behavior; live OpenCode 1.18.9 capability output confirmed build (primary), reviewer-visible transcript evidence was captured, the attempted TUI capture was unreadable due to terminal rendering, and the worktree remained clean.

Evidence: OpenCode worker-agent evidence

Rendered launch behavior: ok - opencode workers name the build agent, receive --model, and omit the unsupported effort axis ok - opencode scouts name the build agent and preserve the selected model ok - opencode secondmates preserve officer agent selection while retaining the selected model OpenCode version: 1.18.9 Build agent listing: build (primary)

Rendered launch behavior:
ok - opencode workers name the build agent, receive --model, and omit the unsupported effort axis
ok - opencode scouts name the build agent and preserve the selected model
ok - opencode secondmates preserve officer agent selection while retaining the selected model

OpenCode version: 1.18.9

Build agent listing:
build (primary)

Evidence: Focused test output

ok - no --model/--effort records defaults and types the claude launch instructions
ok - relative home overrides ignore CDPATH and become absolute before spawn launch construction
ok - FM_HOME defaults resolve relative paths and preserve absolute spellings
ok - absolute override spellings are preserved in spawn launch paths
ok - unresolvable relative spawn overrides fail with named diagnostics
ok - active crew-dispatch profile requires an explicit harness for ship spawns
ok - active crew-dispatch profile requires an explicit harness for scout spawns
ok - active crew-dispatch profile allows an explicit resolved harness
ok - active crew-dispatch profile allows the legacy positional harness form
ok - active crew-dispatch profile allows the raw launch-command escape hatch
ok - claude receives --model and --effort profile flags
ok - codex receives --model and model_reasoning_effort profile flags
ok - codex omits unsupported max effort instead of passing a bad config value
ok - grok receives --model and --reasoning-effort profile flags
ok - grok omits unsupported max reasoning effort
ok - grok omits unsupported xhigh reasoning effort
ok - opencode workers name the build agent, receive --model, and omit the unsupported effort axis
ok - opencode scouts name the build agent and preserve the selected model
ok - opencode secondmates preserve officer agent selection while retaining the selected model
ok - pi receives --model and --thinking max profile flags
ok - pi-signed shares Pi launch semantics while preserving its configured and recorded identity
ok - pi-signed refuses safely and actionably when the selected executable is unavailable
ok - pi-signed is a distinct persistent secondmate runtime with shared Pi supervision semantics
ok - batch dispatch forwards shared --harness, --model, and --effort to every pair
ok - claude forwards firstmate's CLAUDE_CONFIG_DIR so the crewmate uses the same credential store
ok - claude omits the config-dir prefix when firstmate runs with the single-store default
ok - non-claude harnesses do not receive the claude CLAUDE_CONFIG_DIR prefix
ok - active crew-dispatch profile does not block secondmate launches
# all fm-spawn-dispatch-profile tests passed

Pipeline

Updates from git push no-mistakes

⏭️ **intent** - skipped

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

✅ **Review** - passed

✅ No issues found.

✅ **Test** - passed

✅ No issues found.

• bash tests/fm-spawn-dispatch-profile.test.sh
• opencode --version
• timeout 10 opencode agent list | grep '^build '
• Attempted a 160×50 PTY startup capture with opencode --agent build; the raw TUI stream was not reviewer-readable in this terminal environment.
• git status --short confirmed testing left the worktree clean.

✅ **Document** - passed

✅ No issues found.

✅ **Lint** - passed

✅ No issues found.

✅ **Push** - passed

✅ No issues found.

[kunchenguid]

Automated reminder: thanks for the PR! This branch currently has a merge conflict with the base branch.

When you get a chance, please rebase onto (or merge) the latest base branch, resolve the conflict, and push. After that, checks will re-run and the PR will get looked at again.

_{Noted for firstmate#1279 at b6730029.}

[osantis]

Update: the workflows for current head 701e5a0 have completed successfully. All 12 checks pass, and the PR is ready for maintainer review.