feat(agent): persist and report context usage

[jiahe0510]

Summary

• Persist a versioned context_state in session state.json, including current context usage, usage source, compact/microcompact counts, and update time.
• Restore context accounting across restarts and update it after user input, provider responses, tool results, and successful compaction.
• Add /context and /context all with category estimates, and expose AgentEngine::context_status() for SDK hosts.
• Keep existing session files backward-compatible through serde defaults; the effective context window remains runtime configuration and is not persisted.

Why

AionCore needs aionrs to own the effective context window and compaction decisions. Previously, the current context watermark and compaction history were runtime-only, so restarts lost that status and hosts could not query a consistent context view.

Non-goals

• This PR does not add AionCore/AionUi structured context-status events or UI rendering.
• This PR does not introduce model-specific context limits in AionUi.
• This PR does not persist the runtime context window or request-shape fingerprints.

Validation

• just push -u origin jiahe/feat/context-usage
  • cargo fix --allow-dirty --allow-staged
  • cargo clippy --fix --workspace --all-targets --allow-dirty --allow-staged -- -D warnings
  • cargo fmt --all
  • cargo nextest run --workspace --profile default
  • cargo hakari generate --diff
  • cargo hakari manage-deps --dry-run
  • cargo hakari verify
• cargo test -p aion-agent
• cargo check --workspace
• git diff --check

Follow-up

After this lands and is released, AionCore can consume AgentEngine::context_status() and emit a structured context-status event for AionUi.