feat(core): context compression — Focus Agent, SWE-Pruner/COMI, SideQuest (#1850, #1851, #1885)

[bug-ops]

Summary

• Focus Agent (research(context): active context compression via agent-controlled focus primitives (Focus Agent) #1850): native start_focus/complete_focus tools bracket task windows; complete_focus truncates history to UUID-based checkpoint and pins a Knowledge block that survives all compaction
• SWE-Pruner / COMI (research(context): task-aware self-adaptive context pruning (SWE-Pruner) #1851): PruningStrategy enum with TF-IDF–weighted Jaccard scoring and MIG (relevance − redundancy) eviction; task goal cached by user-message hash
• SideQuest (research(context): SideQuest model-driven KV cache management for long-horizon agentic tasks #1885): LLM-driven tool output eviction every K turns with safety guards (eviction ratio cap, pinned-message protection, JSON-parse fallback, focus-active guard)

Critic gap fixes

ID	Fix
S1	compact_context() and prune_tool_outputs() skip focus_pinned messages
S2	TF-IDF weighted Jaccard with Rust/shell stop-word list in compaction_strategy.rs
S3	SideQuest disabled when pruning_strategy != Reactive
S4	complete_focus uses UUID marker; returns error to LLM if marker not found
S5	Task goal cache invalidated by last-user-message hash, not by compaction events

New files

• crates/zeph-core/src/agent/focus.rs — Focus Agent state, tool definitions, Knowledge block management
• crates/zeph-core/src/agent/sidequest.rs — SideQuest cursor tracking and eviction logic
• crates/zeph-core/src/agent/compaction_strategy.rs — TF-IDF scoring and MIG pruning (feature-gated)

Configuration

[agent.focus]
enabled = false
compression_interval = 5
min_messages_per_focus = 3

[memory.sidequest]
enabled = false
interval_turns = 4
max_eviction_ratio = 0.5
max_cursors = 30
min_cursor_tokens = 50

[memory.compression]
pruning_strategy = "reactive"  # reactive | task_aware | mig | task_aware_mig

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace --features full -- -D warnings passes (0 warnings)
• cargo nextest run --workspace --features full --lib --bins — 6027/6027 passed (+35 new unit tests)
• Synced with origin/main (fast-forward, no conflicts)

[bug-ops]

Code review complete (rust-code-reviewer agent). All validator findings verified in code:

• C1/C2 (double-compaction guard): confirmed at native.rs:1576, mod.rs:4146, reset at mod.rs:3135
• S4 (infinite recursion): prune_tool_outputs_oldest_first() extracted at summarization.rs:788
• S5 (checkpoint focus_pinned): confirmed at native.rs:1508
• PERF-1 (SideQuest blocking): two-phase now_or_never() + tokio::spawn at mod.rs:4134/4188
• SEC-CC-01/02/03: all confirmed
• SPDX headers, feature gates, tracing, doc comments, cargo deny — all pass
• Tests: 6043/6043 passed, +16 new tests

Follow-up issue #1904 created for deferred integration points (CLI flags --focus/--sidequest, --init wizard steps, debug dump, TUI spinners).

Ready to merge.

[bug-ops]

Fix pushed (460065a): added #\![cfg_attr(not(feature = "context-compression"), allow(dead_code))] at module level in focus.rs and sidequest.rs. The #[cfg_attr] on struct/impl blocks doesn't propagate to individual fields and methods, causing dead_code errors in ide/server/chat bundles (which exclude context-compression). Module-level attr covers all items.