[FEATURE]: Preserve agent identity across context compaction

[ESRE-dev]

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Describe the enhancement you want to request

Specialized agents (judges, reviewers, planners, explorers) lose their identity after context compaction. The compaction summarizer has no knowledge of which agent is being compacted, so the summary omits role-specific context. Post-compaction, agents revert to generic implementor behavior — e.g. a reviewer starts suggesting code changes instead of reviewing.

Problem:

• The compaction LLM receives no system prompt from the source agent, so it can't preserve role context
• The summary template has no section for agent role/constraints
• After compaction, non-native agents receive a generic "continue" message with no identity reinforcement

Proposed changes:

1. Pass the source agent's system prompt (capped at 4K) to the compaction LLM as a system message
2. Add an "Agent Role & Constraints" section to the compaction summary template
3. Inject a post-compaction <system-reminder> for non-native agents to prevent role drift
4. Expose the agent name on the experimental.session.compacting plugin hook

This is related to #16960 (compaction loses instruction context) and #17900 (agent stops working after compaction), but addresses the runtime agent identity specifically rather than AGENTS.md file preservation.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Improve compaction prompt to preserve critical context details #16512: Improve compaction prompt to preserve critical context details — covers very similar ground (preserving agent constraints, behavioral instructions, and context through compaction), and the proposed solution overlaps significantly with the changes described here.

If your issue is distinct, please clarify how it differs from #16512.

[ESRE-dev]

This is distinct from #16512. That issue is about improving the general compaction prompt to preserve content details (exact values, user-pasted text, debugging state). This issue is specifically about preserving agent identity and role across compaction.

The problem: when a specialized agent (reviewer, judge, planner, explorer) hits compaction, the summarizer has no knowledge of which agent is active. The summary loses all role context, and post-compaction the agent reverts to generic implementor behavior. A reviewer starts suggesting code changes instead of reviewing. A planner starts implementing instead of planning.

What this adds that #16512 does not cover:

1. System prompt forwarding — the source agent's system prompt is passed to the compaction LLM so it understands the role being summarized
2. Agent Role & Constraints template section — a dedicated section in the summary for the agent's name, role, and behavioral constraints
3. Post-compaction identity reminder — a <system-reminder> injected for non-native agents to prevent role drift after context reset
4. Plugin hook enrichment — the agent name is exposed on experimental.session.compacting so plugins can customize compaction per-agent

The two issues are complementary — #16512 improves what the summary contains, this one ensures the summary knows who it's for.