fix: trios-llm missing serde_json dependency (workspace test blocker)

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Problem

crates/trios-llm/src/lib.rs:77 uses serde_json::to_string() but serde_json is not declared in trios-llm/Cargo.toml.

This blocks cargo test --workspace from compiling.

Fix Applied (uncommitted)

Added serde_json.workspace = true to crates/trios-llm/Cargo.toml.

Verification

cargo clippy -p trios-llm -- -D warnings  # PASS
cargo test -p trios-llm                    # 2 passed

Refs: TD-015

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Diagnosed by trios-doctor (leukocyte agent)

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Fix pushed in commit 2eb5911

Added serde_json.workspace = true to crates/trios-llm/Cargo.toml.

Verification:

cargo test -p trios-llm   # 2 passed

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Resolved by PR #120 (merged). serde_json dependency added.

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Fixed by #120 (merged)