fix(agent): restrict memory flush to memory tools only

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Summary

• Pre-compaction memory flush ran a full agent loop under SilentEventSink with builtins/skills/MCP available. Non-key-path write_file needs no confirmation, so a silent flush could overwrite workspace files; Low-tier run_command could also auto-approve after PR fix(agent): deny ConfirmRequired in SilentEventSink #138.
• Flush turns now set memory_tools_only and build a registry that registers only memory_search / memory_write / memory_list (no builtins, skills, or MCP).
• Adds registry regression tests locking the restricted surface.

Task Linkage

• Task ID: TASK-2026-071
• Task folder: tasks/TASK-2026-071-memory-flush-tool-allowlist/

Injected Specs

• spec/architecture-boundaries.md (agent loop / extension registry only; no crate boundary changes)
• spec/security-nonnegotiables.md (silent background turn must not mutate workspace)
• spec/testing-policy.md (Rust code change + regression tests)
• spec/docs-sync.md (reviewed; not needed — no user-facing command/env/docs change)
• Additional: spec/verification-integrity.md, spec/task-artifact-language.md, spec/rust-conventions.md

Validation Evidence

• Commands executed:
  • cargo fmt --check
  • cargo clippy -p skilllite-agent --all-targets -- -D warnings
  • cargo test -p skilllite-agent memory_flush
  • cargo test -p skilllite-agent
  • python3 scripts/validate_tasks.py
• Key results:
  • fmt: exit 0
  • clippy agent: exit 0
  • focused: 2 passed (memory_flush_registry_*)
  • agent crate: 249 passed; 0 failed
  • task validation: Task validation passed (71 task folders checked)

Regression Scope

• Areas likely affected:
  • Pre-compaction / early memory flush turns
  • Extension registry capability policies
• Explicit non-goals:
  • SilentEventSink ConfirmRequired policy (open PR fix(agent): deny ConfirmRequired in SilentEventSink #138)
  • Swarm run_single_task full tool surface
  • ChatSession workspace chat-root alignment

Docs Sync (EN/ZH)

• Not needed
• Updated EN + ZH docs
• Files:
  • N/A

Review Checklist

• Acceptance criteria in tasks/TASK-2026-071-memory-flush-tool-allowlist/TASK.md satisfied
• tasks/TASK-2026-071-memory-flush-tool-allowlist/STATUS.md updated with latest progress
• tasks/TASK-2026-071-memory-flush-tool-allowlist/REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date