Add cache-stable agent capability runtime / 新增缓存稳定的 Agent 能力运行时

[SivanCola]

Summary

• Persist a session RuntimeContract for capability surface, edit protocol, and prompt layout.
• Add stable BM25 capability discovery and proxy execution without mutating provider-visible schemas.
• Add the session-context prompt layout and deterministic three-level compaction state.
• Add opt-in Hashline v1 editing with preview confinement and permission aliases.
• Route dynamic MCP tools to execution and legacy registries while keeping capability schemas stable across connect, disconnect, resume, and lazy cache misses.

Why

Agent sessions previously derived provider-visible tools and prompt layout from process state. Dynamic MCP registration and non-deterministic compaction could change cached prefixes or alter behavior after resume. This change makes those choices session-owned and persisted while preserving the existing default behavior.

Cache and rollout

• Empty configuration still selects legacy-v1, classic-v1, and system-v1.
• Enabling the v2 surface or Hashline intentionally causes one cold prefix when a new session starts.
• Provider-visible capability schemas remain fixed after session creation.
• Cache guard tail averages remain 92–95 percent across all covered dialogue and tool-loop scenarios.
• The default switch remains gated on live provider A/B results.

Compatibility

Area	Existing behavior or data	Result
Pre-contract sidecars	No runtime contract	Pinned to legacy behavior
Missing transcript sidecar	Strict inference, otherwise legacy fallback	Compatible
Corrupt sidecar	Diagnostic plus legacy fallback	Compatible
Session JSON	Optional runtime_contract field	Compatible
Configuration	Missing new fields	Legacy defaults
Provider tool schema	Legacy unchanged; v2 opt-in	Compatible

Validation

• ./scripts/cache-guard.sh
• Root module: go test ./..., go vet ./..., go build ./..., and go test -race ./...
• Desktop module: go test ./..., go vet ./..., go build ./..., and go test -race ./...

Attribution

Hashline is adapted from the xAI grok-build implementation at baseline commit c68e39f under Apache-2.0. The package includes the required license, NOTICE, and adapted-source headers.

Not included

• Switching the product default to the v2 runtime
• Desktop new-task editing protocol controls
• Live provider A/B and the token-reduction release gate

[SivanCola]

Closing this draft in favor of #6705, which has merged into main-v2 as 43993f5.

Adopted in the focused replacement:

• Package-aware MCP/skill runtime bindings and portable-to-canonical tool resolution.
• Canonical permission, approval, hook, evidence, and audit identities.
• Exact-ID-restricted on-demand MCP capability proxies for subagents.
• Live-over-cache precedence, ambiguity rejection, and provider-schema/cache stability guards.

Not adopted from this broader draft:

• The persisted runtime-contract and opt-in v2 capability surface.
• The session-context and three-level compaction redesign.
• Hashline editing and the associated configuration/provider changes.

Those broader experiments remain separate future work. If revisited, they should be split into focused PRs, rebased on current main-v2, and preserve the canonical binding, security-identity, cache-stability, and scoped-proxy contracts now provided by #6705.

Verification for #6705 completed with the full Go test suite, vet, focused race coverage, cache guard, three-platform CI, CodeQL, and desktop builds all passing.

Availability: the focused MCP/skill fix is now on main-v2 and will ship with the next release.