refactor(#2804): model-layer cleanup — RESERVED_MEMORY_TOOL_NAMES reu…

[Skobeltsyn]

…se, named constants, reserveName guard, severity log, AgentEvent dedupe

Five small fixes in the model layer:

Reuse RESERVED_MEMORY_TOOL_NAMES (AgenticLoop.kt)

• The inline setOf("memory_read","memory_write","memory_search") was drifting against the canonical constant in ToolDef.kt. AgenticLoop now imports and uses RESERVED_MEMORY_TOOL_NAMES — adding a 4th memory tool now updates one set, not two.

Named hash-prefix constants (AgenticLoop.kt)

• MANIFEST_HASH_PREFIX_LEN = 12 for the OpenAI prompt_cache_key routing hint suffix.
• BLOB_HASH_PREFIX_LEN = 12 for the content-addressed blob placeholder rendered into tool-result LLM messages.

Named Anthropic constants (ClaudeClient.kt)

• ANTHROPIC_MAX_CACHE_BREAKPOINTS = 4 documents the per-request cap on cache_control markers (Anthropic Messages API limit).
• EPHEMERAL_TTL_BOUNDARY_MINUTES = 5L documents the boundary between the default ephemeral cache and the "ttl":"1h" long-cache tier.

reserveName guard (ToolDef.kt)

• The "Tool already defined in this tools block" require(...) was duplicated 5× across all tool(...) overloads and the unaryPlus operator. Extracted to MutableList<ToolDef>.reserveName(name) — one definition, one place to change if uniqueness semantics evolve.

Severity parse log (ToolDef.kt)

• Severity.valueOf(severityStr) previously fell through to HIGH on any exception (including IllegalArgumentException from an unknown enum value) without telemetry. Now logs at WARNING when the LLM supplies an unknown severity, preserving the safe HIGH default while making the misuse visible.

AgentEvent.ToolCallFinished emit dedupe (AgenticLoop.kt)

• Four near-identical emit blocks (unknown-tool, denied, success, executor exception) replaced with calls to a private emitToolFinished(emitter, agent, call, result, isError) helper.
• Removed all five agents_engine.runtime.events.AgentEvent.… FQN references; the import has been at the top of the file all along.

Full ./gradlew test green; 254+ TEST-*.xml files, zero failures/errors.