feat(edge-worker): forward per-runner custom binary paths to spawned runners

[mitekk]

Motivation

Cyrus's three spawned-binary runners — claude, codex, gemini — each expose an SDK option for replacing the dispatched binary with a custom path:

Runner	SDK option	Default
claude	pathToClaudeCodeExecutable (Anthropic SDK)	bundled CLI
codex	codexPath (cyrus-codex-runner)	codex on PATH
gemini	geminiPath (cyrus-gemini-runner)	gemini on PATH

(The cursor runner is in-process via the Cursor SDK — no spawned binary, so no analogous knob.)

Useful when operators wrap the runner binary for:

• Sandboxed / containerized dispatch (e.g. docker run sandbox …)
• Per-invocation credential brokering or mode switching
• Telemetry header injection or OTEL endpoint rewriting
• Any transform that needs to fire on every runner invocation

Today, none of these options can be set from ~/.cyrus/config.json. cyrus-claude-runner, cyrus-codex-runner, and cyrus-gemini-runner each read their respective option from their own runner config, but nothing upstream of them in cyrus-edge-worker reads from cyrus config and forwards. So operators have to fork cyrus or replace the binary on PATH — neither great.

Change

Adds three optional per-repository fields to RepositoryConfigSchema: pathToClaudeCodeExecutable, codexPath, geminiPath. Each is forwarded by RunnerConfigBuilder.buildIssueConfig to the returned runner config when the resolved runner type matches. Naming matches each SDK's existing option, so the cyrus docs entry maps 1:1 to the SDK docs.

Field placement: per-repository, matching the existing pattern for model, fallbackModel, allowedTools, mcpConfigPath. No top-level fallback (kept the surface minimal; can be added in a follow-up).

Files

• packages/core/src/config-schemas.ts — three optional fields with JSDoc
• packages/edge-worker/src/RunnerConfigBuilder.ts — three runner-type-conditional forwards in buildIssueConfig's claude/codex/gemini blocks
• packages/core/schemas/*.json — regenerated via pnpm --filter cyrus-core generate:json-schema
• docs/CONFIG_FILE.md — documents the three fields under a "Custom runner-binary paths" subsection
• packages/edge-worker/test/RunnerConfigBuilder.runner-binary-paths.test.ts — 9 tests: claude/codex/gemini × forwarded-when-set / omitted-when-unset / ignored-when-wrong-runner

Verification

pnpm build and pnpm test:packages:run both clean on this branch. 632 edge-worker tests pass (was 626 on main, +6 for the new codex/gemini variants); 129 core tests pass; suite-wide green.

Out of scope

• executable field ("bun" vs "node" for claude) — separate concern; wrapper scripts that shebang bash or node work fine via the default.
• Top-level config fallback — kept narrow for review.
• Plumbing in buildChatConfig — only added to buildIssueConfig since that's where the per-runner-type branches live. Small follow-up if you want the same knob for Slack chat sessions.
• Cursor runner — no spawned binary, no analogous SDK option.

Closes no issue — happy to file a separate one if you'd prefer.