feat(memory): Tests, docs, and examples for inject_memories

[hamza-jeddad]

Background

Sub-issue of #3011. Final polish — make sure inject_memories is well-tested, documented, and discoverable via examples once the other sub-issues land.

Scope

Tests

• Unit tests in pkg/hooks/builtins/inject_memories_test.go:
  • Disabled builtin → no-op
  • No memory toolset → no-op
  • Empty memory store → no injection block
  • Local strategy + multi-keyword prompt → correct SearchMemories call
  • max_inject_memories cap respected
  • XML output is well-formed
• Integration test (e.g. under e2e/):
  • Agent with inject_memories: true + a few seeded memories
  • User prompt referencing one memory → that memory appears in the injected block
  • Unrelated user prompt → only fallback / no relevant memories
  • Memory written via tool call → next turn reflects it
• Race detector clean: go test -race ./pkg/hooks/builtins/... ./pkg/runtime/...
• ≥80% line coverage on new code

Docs

• docs/ — new section on proactive memory injection: what it does, how to enable it, when to choose local vs llm strategy
• AGENTS.md — short note explaining inject_memories for agents browsing the repo
• agent-schema.json — confirm descriptions render usefully in IDEs / YAML language servers

Examples

• examples/ — sample agent YAML demonstrating inject_memories: true with a memory toolset
• Optional: a CHANGELOG entry under the next release header

Acceptance Criteria

• All new tests pass on Linux, macOS, and Windows CI
• Coverage report shows ≥80% on new files
• Docs build cleanly (no broken links, lints pass)
• Example agent runs end-to-end via docker-agent run and demonstrably injects memories
• CHANGELOG entry merged with the feature