feat(agent): add explicit context compression tool / 显式上下文压缩工具

[SivanCola]

Summary

• add a model-callable compress tool with exact unique user-message anchors and before / after range semantics
• install all explicit range compaction into context projections without rewriting the canonical transcript
• preserve checkpoint boundaries, rewind, undo, fork, and file recovery across compression
• keep CLI, Desktop, Wails, and HTTP positional compression entrypoints compatible while routing them through the projection core
• update UI copy and tool contract docs to clarify that visible history is retained

Implementation details

• bind a context-scoped ContextCompressor to each tool execution so child agents operate only on their own session projection
• serialize compress calls even though the tool is workspace-read-only and Plan Mode safe
• merge prior compaction summaries, prefer native compaction with summarizer fallback, and reject empty, non-shrinking, or stale projections atomically
• split range planning, hook/archive preparation, projection building, and atomic installation into focused stages
• read schema-v1 sidecars and write schema-v2 sidecars with covered count/hash, prompt-cache lineage, and incremented projection versions
• expose compress in default and Economy tool surfaces while respecting explicit tool allowlists
• surface folded anchors and non-shrinking positional compression as visible warnings instead of silent success

Cache impact

The system prompt is unchanged. Adding the default tool schema changes the tool-prefix hash once after upgrade. Successful explicit compression reports one compact_tool prefix change; noop and failed attempts do not report a rewrite. Schema-v2 downgrade fallback may cause one cold canonical-prefix request while preventing an older client from misreading newer projection invariants.

Validation

• go run ./tools/repolint
• go test ./...
• go test -race ./internal/agent ./internal/control
• go vet ./...
• cd desktop && go test ./...
• cd desktop/frontend && pnpm typecheck
• cd desktop/frontend && pnpm exec tsx src/__tests__/app-chrome-tabs.test.ts
• cd desktop/frontend && pnpm exec tsx src/__tests__/message-reasoning-panel.test.tsx
• git diff --check

Cache-impact: medium - adds one default tool schema after upgrade; successful compression deliberately replaces the model-visible history prefix, while schema-v2 downgrade fallback may cause one cold canonical-prefix request.
Cache-guard: go test ./internal/boot ./internal/agent ./internal/control - existing tool-prefix goldens, compact_tool rewrite assertions, and sidecar compatibility tests cover schema and prefix behavior.
System-prompt-review: system prompt bytes are unchanged; only the stable tool schema set and explicit projection prefix change.
Documentation-impact: updated - documented the compress tool and clarified projection-only context compression while retaining visible history.