Promote issue fields and deprecate legacy issue write tool

[reneexeener]

Summary

This PR removes the remote_mcp_issue_fields feature flag, making issue fields and related functionality (e.g. field_filters support in issue tools) available by default, and updates documentation and tests to reflect this change.

Why

• Fixes https://github.com/github/plan-track-agentic-toolkit/issues/317

What changed

Theme	Change Description	File(s) / Reference(s)
Feature Flag Removal	Remove the FeatureFlagIssueFields (remote_mcp_issue_fields) feature flag and its usage from the codebase and allowed flags list.	pkg/github/feature_flags.go [1] [2]
Documentation Cleanup	Remove conditional documentation for remote_mcp_issue_fields and always document issue field support in issue tools.	docs/feature-flags.md [1] [2] docs/insiders-features.md [3] [4]
Tool Schema Updates	Remove feature-flagged tool schema snapshots for issue fields (*_ff_remote_mcp_issue_fields.snap).	pkg/github/__toolsnaps__/issue_write_ff_remote_mcp_issue_fields.snap [1] pkg/github/__toolsnaps__/list_issues_ff_remote_mcp_issue_fields.snap [2]
Default Tool Expansion	Add issue_fields and field_filters parameters to the default tool schemas and documentation.	README.md [1] [2] [3] pkg/github/__toolsnaps__/issue_write.snap [4] pkg/github/__toolsnaps__/list_issues.snap [5]
Test Adjustments	Update tests to use a different feature flag (FeatureFlagFileBlame) for CSV output gating, reflecting the removal.	pkg/github/csv_output_test.go

These changes streamline the codebase by making issue field support a standard feature, reducing complexity and improving documentation clarity.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)