Suppress non-fatal OpenDAL memory backend warnings

Description

When running terraphim-agent graph or terraphim-agent replace, multiple warning messages are logged even though the operations complete successfully:

[2026-01-06T18:17:40Z WARN  opendal::services] service=memory name=0x60000018c910 path=embedded_config.json: read failed NotFound (permanent) at read, context: { service: memory, path: embedded_config.json, range: 0- } => memory doesn't have this path
[2026-01-06T18:17:54Z WARN  opendal::services] service=memory name=0x600003bec9d0 path=thesaurus_terraphim_engineer.json: read failed NotFound (permanent) at read
[2026-01-06T18:17:54Z ERROR terraphim_service] Failed to load thesaurus: OpenDal(NotFound (permanent) at read => memory doesn't have this path

Impact

Functional: None - replacements work correctly despite warnings
UX: Confusing for users who see ERROR/WARN logs for successful operations
Git hooks: Warnings pollute hook output (currently suppressed with 2>/dev/null)

Expected Behavior

Optional config files should not produce warnings when missing
The thesaurus_*.json loading should gracefully handle "not found" without ERROR level log
Or: provide a way to configure log level / suppress expected "not found" cases

Environment

terraphim-agent version: latest from cargo install
OS: macOS Darwin 23.6.0
Usage: git prepare-commit-msg hook for knowledge graph replacements

Reproduction

terraphim-agent graph --role "Terraphim Engineer"
echo "test" | terraphim-agent replace --role "Terraphim Engineer"

Suggested Fix Options

Change log level from WARN/ERROR to DEBUG for optional file lookups
Check file existence before attempting read
Add configuration option to suppress expected "not found" warnings
Use Result handling that distinguishes "missing optional file" from "actual error"

Phase 4 Verification Report

Status: NOT FIXED

Evidence

Testing on 2026-01-17 still shows warnings:

[WARN opendal::services] service=memory path=embedded_config.json: read failed NotFound (permanent)
[WARN opendal::services] service=memory path=thesaurus_terraphim_engineer.json: read failed NotFound (permanent)
[ERROR terraphim_service] Failed to load thesaurus: OpenDal(NotFound ...)

Root Cause

OpenDAL Internal Logging: Logs WARN level when read() fails with NotFound (expected for optional files)
Terraphim Service: Uses log::error! at lib.rs:462 before attempting fallback