Pack v2: resolve agent-local append_fragments spec/runtime gap

[donbox]

Summary

The reconciled Pack v2 prompt spec treats agent-local append_fragments as meaningful, but the current runtime only wires [agent_defaults].append_fragments cleanly. Agent-local prompt fragment appends are still routed through older inject_fragments behavior.

Why

This surfaced during migration-validation-suite-first-pass. We do not want to silently weaken the spec, but we also do not want to invent product behavior inside the validation suite. The gap should be tracked explicitly and resolved deliberately.

Current behavior

• [agent_defaults].append_fragments affects rendered prompts
• agent-local fragment appends are not honored under the same contract
• runtime still uses inject_fragments paths in the agent-local flow

Decide here

• implement agent-local append_fragments support to match the reconciled spec
• or explicitly downgrade/document the expectation so the suite and docs agree

Evidence

• internal/config/config.go
• cmd/gc/agent_build_params.go
• cmd/gc/template_resolve.go
• task: migration-validation-suite-first-pass

Notes

Per current triage philosophy, this is logged as debt/breadcrumb unless it proves destabilizing to key flows.