feat: add file-based pattern support

[mattjoyce]

What this Pull Request (PR) does

Add file-based pattern support

This PR adds the ability to load patterns directly from files using explicit path prefixes, making it easier to test and iterate on patterns without requiring installation into the fabric config structure.

Why

Currently, to test a pattern it needs to be instantiated in the fabric config file structure. This change allows for quicker testing and iteration of patterns directly from files.

What

• Adds support for loading patterns from files using explicit path prefixes
• Maintains full backwards compatibility with named patterns
• Handles common path formats:
  • Relative paths (./pattern.txt, ../pattern.txt)
  • Home directory (~/patterns/test.txt)
  • Absolute paths (/path/to/pattern.txt)

How

Use explicit path prefixes to distinguish file paths from pattern names:

# From files
fabric --pattern ./draft-pattern.txt
fabric --pattern ~/patterns/my-pattern.txt
fabric --pattern ../../shared-patterns/test.txt

# Existing pattern behavior remains unchanged
fabric --pattern patternname

[eugeis]

Hi, thank you for the PR.

Please 1) move the impl. from chatter to in Patterns struct 2) implement variable replacement

[mattjoyce]

@eugeis , you want me to move the new code, and the existing pattern code in chatter.go to pattern.go?

I did implement variable substitution in the new code, line 93 in the pattern.go commit. I think I understand though, we can do variable substitution once. yes?

[mattjoyce]

@eugeis , I have refactored as you asked, but you merged too quickly! :) I have added the new changed to new PR. #1147