feat(agents): claude xml task_guidance prompt residual

[TheGreatAxios]

Summary

• Claude-family leaf workers append one balanced <task_guidance> XML block once at the tail of the system prompt via the ModelFamilyPolicy promptResidual seam (isClaudeLeafProvider + detectModelFamily claude row + CLAUDE_POLICY + buildClaudeTaskGuidanceNote + run.ts wiring).
• Balanced tags; leaf-only (orchestrators resolve to the permissive default, no residual); never applied to grok/gpt rows; never a full-prompt XML renderer; no director fork.
• Overlap flag: blocked-lane CL-8297 (parallel promptResidual lane, NOT merged) was assumed absent, so this unit adds the minimal local promptResidual hook in buildSubAgentSystemPrompt. If CL-8297 lands a generic seam first, this field should be subsumed by it on rebase.

Verification

• RED then GREEN on the same branch: 875689f (failing tests) then 11fd819 (implementation).
• bun test src/subagent/provider-family.test.ts src/agent/model-family-policy.test.ts src/agent/prompts.test.ts: 53 pass, 0 fail.
• bun test src/agent src/subagent: 1356 pass, 0 fail.
• bun run lint (oxfmt + oxlint): clean.
• bun run typecheck: 1 error, pre-existing and unrelated (vendor/intx-types semver @types, file untouched by this change).

Fixes CL-8309

[linear-code]

CL-8309