feat: persistent CLI configuration with layered resolution

[broomva]

Summary

• Add TOML-based two-layer config system: file-based config for pre-daemon bootstrap (provider, model, port), Lago SessionManifest for post-daemon agent context
• New CLI commands: arcan config init/set/get/list, arcan status, arcan stop
• New global flags --provider and --model; --port now optional with config fallback
• Resolution chain: hardcoded defaults → global config (~/.config/arcan/config.toml) → project-local config (.arcan/config.toml) → env vars → CLI flags
• Provider construction refactored with from_resolved() constructors that accept config overrides while always reading API keys from env vars (never from config file)
• 10 new tests for config module (merge, set/get, resolve, serialization roundtrip, save/load)

Example Usage

# One-time setup
arcan config init
arcan config set provider ollama
arcan config set model gpt-oss:20b
arcan config set port 3001

# Now just:
arcan run "What is 2+2?"                       # uses config defaults
arcan run "Hello" --provider mock              # CLI flag overrides
ARCAN_PROVIDER=anthropic arcan run "Hello"     # env var overrides

# Daemon management
arcan status                                   # provider, model, port, daemon status
arcan stop                                     # graceful shutdown

Files Changed

File	Change
NEW: crates/arcan/src/config.rs	Config structs, loading, merging, saving, ResolvedConfig
crates/arcan/src/main.rs	Add mod config, global flags, Config/Status/Stop commands, refactored provider selection
crates/arcan/src/daemon.rs	Forward provider/model to daemon spawn, public helpers, stop_daemon()
crates/arcan/src/cli_run.rs	Accept optional model param, propagate into session creation
crates/arcan/Cargo.toml	Add toml, dirs deps
Cargo.toml (workspace)	Add toml, dirs to workspace deps
crates/arcan-provider/src/anthropic.rs	Add from_resolved() constructor
crates/arcan-provider/src/openai.rs	Add openai_from_resolved(), ollama_from_resolved() constructors

Test plan

• cargo fmt && cargo clippy --workspace -- -D warnings passes
• cargo test --workspace — 246 tests passing (+1 ignored)
• cargo build --workspace succeeds
• Manual: arcan config init && arcan config set provider mock && arcan config list
• Manual: arcan status shows resolved config
• Manual: arcan run "Hello" --provider mock (CLI override)
• Manual: arcan stop graceful shutdown

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