fix(gui): implement missing Tauri backend commands for action gap fixes

[echobt]

Summary

This PR addresses the Action Gaps audit findings for the cortex-gui IDE, implementing 16 missing backend commands that were invoked from the frontend but had no corresponding Tauri command handler.

Problem

The previous audit identified that many UI elements in the IDE were non-functional because they invoked backend commands that didn't exist. This caused silent failures when users:

• Used search and replace functionality
• Executed notebook cells
• Tried to terminate processes from the Process Explorer
• Used git operations like branch rename, reset, or clean

Solution

New Modules

search.rs - Search and replace functionality

• search_replace_all: Replace matches across multiple files
• search_replace_in_file: Replace all matches in a single file
• search_replace_match: Replace a single match

notebook.rs - Notebook kernel execution

• notebook_execute_cell: Execute code in a notebook cell
• notebook_interrupt_kernel: Interrupt running kernel
• notebook_shutdown_kernel: Shutdown a kernel
• notebook_start_kernel: Start a new kernel for a notebook

process.rs - Process management

• terminate_cortex_process: Kill processes with Unix/Windows support

Git Enhancements (git.rs)

• git_branch_rename: Rename a local branch
• git_reset_soft: Soft reset to a commit (keeps changes staged)
• git_clean: Clean untracked files
• git_stage_lines: Stage specific lines from a file
• git_unstage_lines: Unstage specific lines from a file
• git_remote_add/remove/rename: Aliases for frontend naming conventions

Testing

• Cargo check passes successfully
• All new commands are properly registered in lib.rs
• Code formatted with cargo fmt

Audit Report

The updated AUDIT_REPORT.md now reflects the status of all fixes and remaining work.

Breaking Changes

None - all changes are additive.