refactor: remove P generic from Agent, SemanticMemory, CodeRetriever (M24)

[bug-ops]

Summary

Removes the P: LlmProvider generic parameter from core types, completing the M24 architecture improvements epic.

Architecture (#398, #407)

• Agent<P, C, T> -> Agent<C, T> — stores AnyProvider directly
• SemanticMemory<P> -> SemanticMemory — stores AnyProvider directly
• CodeRetriever<P> -> CodeRetriever, CodeIndexer<P> -> CodeIndexer
• MemoryState, IndexState lose P parameter
• Eliminates N monomorphizations, reduces binary size and compile time

Performance (#400, #402)

• perf: reduce provider clone overhead in skill embedding #400 resolved: AnyProvider clone is cheap (enum), no Arc needed
• cached_prompt_tokens field with incremental updates on message push/pop/drain/compact

Security (#414)

• Safetensors header validation (file size, header bounds, JSON parse) before unsafe mmap

Developer experience (#406)

• sccache setup guide added to mdBook docs

Testing

• Centralized MockProvider in zeph-llm under mock feature flag
• Replaces 3 ad-hoc test providers across crates

Stats

• 21 files changed, 407 insertions, 345 deletions (-net reduction)
• 1533 tests pass, 0 failures
• Clippy clean, fmt clean

Breaking changes

• Agent<P, C, T> -> Agent<C, T>
• SemanticMemory<P> -> SemanticMemory
• CodeRetriever<P>, CodeIndexer<P>, IndexWatcher<P> lose P
• Test code must use mock feature for MockProvider

Test plan

• cargo nextest run --workspace --lib --bins --features full,mock
• cargo clippy --workspace --features full -- -D warnings
• Code review approved
• CI gate passes

Resolves #398, #400, #402, #406, #407, #414
Completes epic #391