Fix non-terminating stash retrieve loop on explicit working-memory reads

[rockfordlhotka]

Problem

The 2026-06-10 10am communications briefing cron job spent 15 minutes and timed out. Investigation of the live k8s cluster showed calendar-mcp was healthy and fast (yesterday's IMAP fix #69 / image 1.4.1 works — email/calendar calls complete in 1–6s). The timeout was a non-terminating stash retrieve loop in the agent host.

When a tool result is too large, the host trims it (head + elision marker + tail) and stashes the full original in working memory under stash/{session}/{callId}, telling the model to fetch it via GetFromWorkingMemory. But the retrieval result was itself oversized, so the per-call cap (CapToolResultAsync) and watermark trimmer (ToolResultTrimmer) re-stashed it under the retrieval call's new id and advertised that key back. The model fetched it, got a larger reference, which was re-stashed again — looping ~35s/iteration until the budget killed it:

GetFromWorkingMemory(stash/.../call_iz7u7s2) -> big -> re-stashed as call_V5QImO2
GetFromWorkingMemory(stash/.../call_V5QImO2) -> big -> re-stashed as call_Qm6TZL
GetFromWorkingMemory(stash/.../call_Qm6TZL)  -> ... (until 15-min timeout)

The earlier llm-high-tier-cost-guard subagent hit the identical loop.

Fix

ChunkingAIFunction already exempted the working-memory read tools (GetFromWorkingMemory/SearchWorkingMemory/ListWorkingMemory) from re-chunking for exactly this reason. This PR centralizes that exemption in a shared StashExemptTools set and honors it in all three paths:

• StashExemptTools (new) — single source of truth.
• ChunkingAIFunction — uses the shared set (removed private duplicate).
• CapToolResultAsync — returns explicit-retrieval results unchanged.
• ToolResultTrimmer.TrimAsync — skips exempt results when picking the largest result to trim.

An explicit retrieval is now always returned in full and never re-stashed.

Tests

Added 4 regression tests (with [Timeout] guards mirroring the real loop). RockBot.Host.Tests: 1061 passed, 0 failed.

Deployment

Version bumped 0.12.29 -> 0.12.30. Image rockylhotka/rockbot-agent:0.12.30 built, pushed, and deployed to the live rockbot namespace via kubectl set image for testing; calendar-mcp confirms Client (RockBot.Agent 0.12.30.0) is live.