feat: add function ID resolution like session ID

[giordano-lucas]

Add NOTTE_FUNCTION_ID env var and current_function file support to functions commands, mirroring the existing session ID pattern.

Resolution priority: --id flag > NOTTE_FUNCTION_ID env > current_function file

Changes:

• Add CurrentFunctionFile and EnvFunctionID constants to config
• Add GetCurrentFunctionID, RequireFunctionID, setCurrentFunction, clearCurrentFunction functions
• Update all function commands to use RequireFunctionID
• Create command saves function ID to current_function file
• Delete command clears current_function if matching
• Add comprehensive unit tests for function ID resolution
• Add integration tests for functions including ID resolution

Greptile Overview

Greptile Summary

This PR adds function ID resolution with priority: --id flag > NOTTE_FUNCTION_ID env var > current_function file. The implementation mirrors the existing session ID pattern consistently.

Key Changes

• Added GetCurrentFunctionID(), RequireFunctionID(), setCurrentFunction(), and clearCurrentFunction() helper functions
• Updated all function commands to use RequireFunctionID() instead of requiring --id flag
• Create command saves function ID to current_function file
• Delete command clears current_function file only if it matches the deleted function
• Added comprehensive unit tests (11 new tests) covering priority resolution, file operations, and edge cases
• Added integration tests (10 new tests) covering end-to-end workflows and ID resolution scenarios

The implementation is clean, well-tested, and follows established patterns in the codebase.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Implementation mirrors existing session ID pattern exactly, has comprehensive test coverage (unit and integration), and follows all established coding conventions
• No files require special attention

Important Files Changed

Filename	Overview
internal/cmd/functions.go	Added function ID resolution pattern (flag > env > file) mirroring sessions, comprehensive and well-implemented
internal/cmd/functions_test.go	Added comprehensive unit tests covering all ID resolution scenarios and edge cases
internal/config/config.go	Added CurrentFunctionFile and EnvFunctionID constants following existing naming conventions
tests/integration/functions_test.go	Added new integration test file with comprehensive end-to-end tests for function lifecycle and ID resolution

[greptile-apps]

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