fix: Centralize filename regex into shared patterns module

[Copilot]

grok_bridge.py and grok_agent.py had divergent filename_pattern regexes — the bridge included jsx|tsx|sh|bash while the agent did not, causing inconsistent code-block parsing across the two modules.

Changes

• src/shared/patterns.py (new): Canonical SUPPORTED_EXTENSIONS string and get_filename_pattern_string() returning the unified regex
• src/shared/__init__.py (new): Package marker for the shared module
• Both grok_bridge.py and grok_agent.py were already updated (prior commits) to import from this module; this PR supplies the missing implementation

Result

# src/shared/patterns.py
SUPPORTED_EXTENSIONS = (
    "py|js|ts|jsx|tsx|go|rs|java|c|cpp|h|hpp|cs|rb|php|swift|kt|scala|sh|bash"
)

def get_filename_pattern_string() -> str:
    return r"^\s*#\s*([a-zA-Z_][a-zA-Z0-9_]*\.(?:" + SUPPORTED_EXTENSIONS + r"))\s*$"

Both modules now compile the identical regex from a single source; extension list changes need to be made in exactly one place.

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