refactor: Clean up Cargo dependencies - remove unused, update outdated

- Remove unused const_format from g3-planner (never imported)
- Remove unused thiserror from workspace and 5 crates (declared but never used)
- Update termimad 0.31 -> 0.34 in studio (consistency with g3-cli)
- Update indicatif 0.17 -> 0.18 in g3-cli
- Update ratatui 0.29 -> 0.30 in g3-cli
- Update walkdir 2.4 -> 2.5 in g3-core
- Update image 0.24 -> 0.25 in g3-computer-control (macOS + Linux)
- Update config 0.14 -> 0.15 in workspace

Blocked: reqwest 0.11 -> 0.12/0.13 requires breaking API changes to
bytes_stream() used in 4 providers - needs separate migration effort.

All tests pass. No behavior changes.

Agent: fowler

crates/g3-providers/Cargo.toml

@@ -8,7 +8,6 @@ description = "LLM provider abstractions for G3 AI coding agent"
 tokio = { workspace = true }
 reqwest = { workspace = true }
 anyhow = { workspace = true }
-thiserror = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 tracing = { workspace = true }