ADR-021: Memory plane and pluggable recall

[mostlydev]

Summary

Stacked on top of #92.

• adds ADR-021 for the ambient memory plane as a compiled capability
• defines how memory fits ADR-020 without collapsing into feeds or tools
• keeps session history as the immutable substrate from ADR-018
• adds the companion implementation plan for pluggable recall and governed retention

Review context

This PR is intentionally scoped to docs only:

• docs/decisions/021-memory-plane-and-pluggable-recall.md
• docs/plans/2026-03-30-memory-plane-and-pluggable-recall.md

Key decisions:

• memory is a sibling compiled capability alongside feeds and tools
• ambient recall/retain are mediated by cllama in v1
• explicit memory tools may still exist, but they are complementary
• backfill is first-class and ADR-018 Phase 2 is a prerequisite for a supported rollout
• operators should prefer one ambient memory plane rather than mixing infra memory with runner-native injection

Test plan

• Team review of ADR-021 shape and its dependency on ADR-020
• Confirm descriptor/context evolution should land in one implementation wave
• Confirm stacked sequencing vs waiting for ADR-020: Compiled Tool Mediation via cllama #92 merge