## ADDED Requirements

### Requirement: Knowledge retrieval SHALL keep L0 as a hint provider
The `lookup_knowledge` retrieval flow SHALL retain L0 keyword/frontmatter matching but use it as domain, entity, and explainability hint data rather than as the sole final retrieval decision.

#### Scenario: L0 produces traceable hint data
- **WHEN** L0 matches one or more indexed knowledge entries
- **THEN** the retrieval flow SHALL expose matched titles, matched keywords, domains or categories, and entity terms as structured hint data

#### Scenario: L0 does not bypass semantic retrieval by default
- **WHEN** L0 returns exactly one match
- **THEN** the retrieval flow SHALL still attempt semantic L1 retrieval unless L1 is unavailable or explicitly disabled by configuration

### Requirement: Knowledge retrieval SHALL use L0 domain as optional L1 filter
The retrieval flow SHALL use L0 domain/category information as an optional metadata filter for L1 retrieval when the domain is unambiguous.

#### Scenario: Single domain filter
- **WHEN** L0 hint data contains exactly one nonblank domain or category
- **THEN** the L1 retrieval request SHALL include that category as a metadata filter

#### Scenario: Ambiguous domain fallback
- **WHEN** L0 hint data contains zero domains or multiple domains
- **THEN** the L1 retrieval request SHALL run without an L0-derived category filter

### Requirement: Knowledge retrieval SHALL preserve fallback evidence
The retrieval flow SHALL still return useful L0 evidence when L1 produces no usable result.

#### Scenario: L1 has no results
- **WHEN** L0 has at least one match and L1 returns no candidates
- **THEN** the tool SHALL return an L0-based primary result
- **AND** the relevance assessment SHALL not claim semantic support from L1

#### Scenario: L1 fails
- **WHEN** L0 has at least one match and L1 retrieval throws or fails
- **THEN** the tool SHALL return an L0-based primary result
- **AND** the tool invocation record SHALL preserve the L0 hint details

### Requirement: Knowledge retrieval SHALL persist L0 hints
The system SHALL persist L0 hint details in `tool_invocation.retrieval_details` for `lookup_knowledge` calls.

#### Scenario: Retrieval details include L0 hints
- **WHEN** a `lookup_knowledge` call records a tool invocation
- **THEN** `retrieval_details` SHALL include L0 matched keywords, domains, entities, and titles when available

#### Scenario: Retrieval layer reflects cooperating retrieval
- **WHEN** both L0 hint data and L1 candidates participate in a lookup
- **THEN** the recorded retrieval layer SHALL be `L0+L1`