♻️ Unify external tool invocations behind shared ExternalTool trait

[renardeinside]

Summary

• Introduces a shared ExternalTool trait and crates/core/src/external/ module with per-tool implementations (Uv, UvTool, Bun, Git, Gh, DatabricksCli)
• Replaces ad-hoc UvCommand, BunCommand, ApxCommand structs and raw Command::new() calls across ~20 files with consistent abstractions
• Adds CommandOutput value type and CommandError enum (via thiserror) replacing repeated stdout/stderr destructuring and ad-hoc error strings
• Merges duplicate get_databricks_sdk_version() / get_databricks_sdk_version_for_project() into a single function with optional project_dir parameter

Changes

Area	Before	After
Command abstraction	Independent UvCommand/BunCommand/ApxCommand + raw Command::new()	Shared ExternalTool trait + per-tool structs
Error handling	Mix of Result<_, String>, FeedbackError, raw status checks	CommandError enum with NotFound, Spawn, Failed, Timeout variants
Output handling	String::from_utf8_lossy(&output.stdout) repeated 15+ times	CommandOutput with .check() and .into_stdout() helpers
Git operations (init.rs)	5x Command::new("git") + helper functions	Git struct with init(), add(), commit(), is_inside_work_tree()
Databricks CLI (MCP)	170-line monolithic handler	DatabricksCli::apps_logs() + thin handler
SDK version check	Two near-identical functions	Single get_databricks_sdk_version(project_dir: Option<&Path>)

New files

• crates/core/src/external/mod.rs — CommandOutput, CommandError, ExternalTool trait, run_command/run_command_sync
• crates/core/src/external/uv.rs — Uv, UvTool, ApxTool
• crates/core/src/external/bun.rs — Bun
• crates/core/src/external/git.rs — Git
• crates/core/src/external/gh.rs — Gh
• crates/core/src/external/databricks.rs — DatabricksCli, AppsLogsArgs, AppsLogsResult

Test plan

• cargo build --workspace — compiles cleanly
• cargo clippy --workspace — no warnings
• cargo test --workspace — all 337 tests pass
• cargo fmt --all -- --check — formatted

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