refactor: Clean up Cargo dependencies - remove unused, update outdated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-core/Cargo.toml

@@ -15,7 +15,6 @@ g3-computer-control = { path = "../g3-computer-control" }
 tokio = { workspace = true }
 reqwest = { workspace = true }
 anyhow = { workspace = true }
-thiserror = { workspace = true }
 tracing = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
@@ -45,7 +44,7 @@ tree-sitter-haskell = { git = "https://github.com/tree-sitter/tree-sitter-haskel
 tree-sitter-scheme = "0.24"
 tree-sitter-racket = "0.24"
 streaming-iterator = "0.1"
-walkdir = "2.4"
+walkdir = "2.5"
 
 # Datalog engine for invariant verification
 datafrog = "2.0.1"