Add continuous compaction with contextual hand summaries

[pbranchu]

Closes #896

Summary

Addresses the core problem in #896: conversation history grows unboundedly, old topics bleed into new sessions, and the agent has no situational awareness of what happened between exchanges.

What this adds

Continuous compaction — every N exchanges (configurable, default 5), when the session exceeds keep_recent messages:

1. Summarizes older turns into a compact paragraph
2. Queries configured hands (calendar, mail, etc.) for a time-bounded context summary
3. Injects the context into the live session as a message — visible on the next LLM turn without requiring memory_recall

Session gap detection — when a user returns after a configurable gap (default 15 min), the bridge triggers compaction + context refresh before dispatching the first message. The context preamble is prepended so the agent is situationally aware on the first response.

Key design points

• Config-driven context sources: only hands explicitly listed under [[compaction.context_sources]] are queried — no unexpected calls to arbitrary hands
• Bounded time window: queries use from_ts (last compaction) → to_ts (now), tracked per-agent; max_lookback_secs caps how far back a gap query reaches
• Parallel queries: all context sources run concurrently via tokio::spawn, latency is the slowest source not the sum
• Session injection: context appears as a [Context refresh — timestamp] user message appended after compaction, in the keep_recent window
• Streaming and non-streaming paths: both paths share the same query_context_sources_parallel() and inject_context_into_session() helpers

Configuration

[compaction]
continuous_interval = 5     # compact every N exchanges (0 = disabled)
keep_recent = 6             # verbatim turns to keep
session_gap_secs = 900      # gap that triggers pre-dispatch refresh (0 = disabled)
max_lookback_secs = 86400   # cap on gap query lookback

[[compaction.context_sources]]
hand = "calendar-hand"
prompt = "Summarize events from the last few hours and any upcoming in the next 24 hours. Be concise."

[[compaction.context_sources]]
hand = "mail-hand"
prompt = "Summarize unread or notable emails. Exclude newsletters and advertisements."

Also fixes

• StreamableHttpClientTransportConfig struct expression broken by rmcp 1.3.0 marking it #[non_exhaustive] — switched to the with_uri().custom_headers() builder API
• Two pre-existing clippy warnings: is_multiple_of() and redundant closure in LazyLock

Test plan

• cargo clippy -- -D warnings passes
• cargo test passes
• With continuous_interval = 5: after 5 exchanges, compaction triggers and context appears in session
• Context source hands receive from {ts} to {ts} time window in query
• Sources queried in parallel (check logs show concurrent responses)
• Session gap: return after >15 min, first response reflects calendar/mail context
• max_lookback_secs caps gap query (3-day absence doesn't dump 3 days of events)
• Empty context_sources list: compaction runs normally, no hand queries
• Hand timeout (30s): skipped gracefully, other sources still collected

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