feat(runtime): smarter handling of large tool outputs (opt-in middle-out truncation)

[aheritier]

Summary

Add smarter truncation for oversized tool outputs. Today max_old_tool_call_tokens is opt-in and only replaces OLDER tool results wholesale with "[content truncated]"; there is no cap on a single large tool output at ingest and no middle-out truncation.

Proposed behavior

• Add a per-result cap with middle-out truncation (keep head + tail, drop the middle) for oversized single tool outputs at ingest.
• Decision: keep defaults OFF — ship this opt-in first. A non-zero default for max_old_tool_call_tokens is explicitly deferred (would be a behavior change for existing users).

Implementation notes (starting points)

• pkg/session/session.go (truncateOldToolContent at ~1290-1293, 1536-1540; add an ingest-time cap)
• Config key(s) in pkg/config/latest/types.go + agent-schema.json + example YAML
• Must NOT weaken the kept-suffix turn-boundary logic (SplitIndexForKeep in pkg/compaction)

Complexity: M.

Background: shared plan context-management-gap-analysis (Phase 2, item 5).

Dependencies / Relationships

• Independent — no dependencies on the other context-management issues.