feat: add allow_workflow_modifications input

[isc]

Summary

Add an optional allow_workflow_modifications input (defaults to false) that removes the prompt-level restriction preventing Claude from modifying .github/workflows/ files.

Problem

The system prompt in generateDefaultPrompt() unconditionally tells Claude:

You CANNOT modify files in the .github/workflows directory

This makes sense for Anthropic's hosted GitHub App, but organizations using custom GitHub App tokens with workflows:write have no way to lift this restriction. The prompt contradicts the actual token permissions, causing Claude to refuse operations it could successfully perform.

See #1282 for full context.

Changes

File	Change
action.yml	Add allow_workflow_modifications input + pass as env var
src/github/context.ts	Parse the new input into BaseContext.inputs
src/create-prompt/index.ts	Conditionally omit the workflow restriction line (follows the existing includeFixLinks pattern)
test/**	Add allowWorkflowModifications: false to all test fixtures

Behavior

• Default (false): No change — Claude still sees the "CANNOT modify workflows" instruction
• true: The restriction line is omitted from the prompt, allowing Claude to modify workflow files if the token permits it

Testing

• TypeScript typecheck passes (bun run typecheck)
• All existing test fixtures updated
• Validated end-to-end at Doctolib: Claude successfully modified a workflow file, committed, and pushed when given workflows:write permission

Closes #1282