Fix #3: terraphim_spawner SpawnContext for per-call working_dir

[AlexMikhalev]

Refs terraphim/adf-fleet#3

Summary

• Add SpawnContext struct with global(), with_working_dir(), and with_env() constructors
• Thread ctx: SpawnContext through all public spawn methods and internal helpers
• spawn_process now uses ctx.working_dir (highest priority) over Configuration.working_dir over spawner default; applies ctx.env_overrides after spawner and provider env vars
• Migrated orchestrator callers to pass SpawnContext::global() preserving existing behaviour
• 4 new code_review_checklists: global preserves defaults, working_dir override, env override, inherited env flows through

Test plan

• cargo test -p terraphim_spawner -- 53 passed, 0 failed
• cargo build --workspace -- clean
• cargo clippy -p terraphim_spawner -p terraphim_orchestrator -- -D warnings -- clean
• cargo fmt --check -- no diff

Notes

Pre-existing clippy warnings in terraphim_agent (unrelated dead code) not introduced by this PR. Orchestrator callers use SpawnContext::global() for now; issue #4 will wire real per-project context.