feat: gate issue_write and get_issue behind remote_mcp_issue_fields flag

[kelsey-myers]

Summary

Gates issue_write and get_issue field enrichment behind the remote_mcp_issue_fields feature flag. Stacks on top of #2551.

Extends the inventory FeatureFlagEnable/FeatureFlagDisable fields from string to []string (AND semantics for enable, OR for disable) to avoid 4-way tool duplication when gating against multiple flags simultaneously.

Why

issue_write exposes issue_fields (Issues 2.0) and get_issue runs an extra GraphQL round-trip for field values — both should only happen when the flag is on, consistent with how list_issues and search_issues were gated in #2520.

What changed

• FeatureFlagEnable/FeatureFlagDisable on ServerTool, ServerResourceTemplate, ServerPrompt changed from string to []string; featureFlagAllowed updated with range loops (enable = all must pass, disable = any blocks)
• LegacyIssueWrite: new FeatureFlagDisable = []string{FeatureFlagIssuesGranular, FeatureFlagIssueFields} variant — no issue_fields schema param, passes nil field values
• IssueWrite: gains FeatureFlagEnable = []string{FeatureFlagIssueFields} — only active when flag is on
• get_issue field_values enrichment wrapped in IsFeatureEnabled(ctx, FeatureFlagIssueFields) runtime check
• New toolsnaps: issue_write.snap (legacy) + issue_write_ff_remote_mcp_issue_fields.snap (flag-enabled), following feat(issues): gate issue-fields features behind remote_mcp_issue_fields flag #2520 convention
• featureFlagAllowed disable loop modernised to slices.ContainsFunc

MCP impact

• Tool schema or behavior changed — issue_write schema splits into two registrations; issue_fields param only visible when flag is on

Prompts tested (tool changes only)

• "Create an issue in github/planning-tracking-demo with Priority P2 and Visibility Medium" → issue_write with issue_fields ✅
• "Get issue 1992 in github/planning-tracking-demo" → get_issue returns field_values: [{Priority: P2}, {Visibility: Medium}] ✅

Security / limits

• No security or limits impact

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

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

Docs

• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR gates issue-field-related issue_write schema/behavior and get_issue enrichment behind remote_mcp_issue_fields, while expanding inventory feature flag annotations to support multiple flags.

Changes:

• Converts inventory feature flag annotations from single strings to string slices with AND enable / OR disable semantics.
• Splits issue_write into legacy and issue-fields-enabled variants with matching toolsnaps and docs.
• Adds a runtime feature check before get_issue GraphQL field-value enrichment.

Show a summary per file

File	Description
README.md	Removes issue_fields from default issue_write docs.
pkg/inventory/server_tool.go	Updates tool feature flag fields to slices.
pkg/inventory/resources.go	Updates resource feature flag fields to slices.
pkg/inventory/prompts.go	Updates prompt feature flag fields to slices.
pkg/inventory/filters.go	Implements multi-flag filtering semantics.
pkg/inventory/registry_test.go	Updates inventory tests for slice-based feature flags.
pkg/http/handler_test.go	Updates HTTP inventory tests for slice-based feature flags.
pkg/github/tools.go	Registers the legacy issue_write variant.
pkg/github/tools_validation_test.go	Updates duplicate-tool validation for slice flags.
pkg/github/pullrequests.go	Migrates PR consolidated tools to slice disable flags.
pkg/github/pullrequests_granular.go	Migrates PR granular tools to slice enable flags.
pkg/github/issues.go	Gates get_issue enrichment and adds legacy/flagged issue_write.
pkg/github/issues_test.go	Updates issue tool snapshots/tests for split variants.
pkg/github/issues_granular.go	Migrates issue granular tools to slice enable flags.
pkg/github/issue_fields.go	Migrates issue-fields tool to slice enable flags.
pkg/github/granular_tools_test.go	Updates granular tool tests for slice flags.
pkg/github/csv_output_test.go	Updates CSV wrapper tests for slice flags.
pkg/github/toolsnaps/issue_write.snap	Updates base issue_write schema to legacy shape.
pkg/github/toolsnaps/issue_write_ff_remote_mcp_issue_fields.snap	Adds flagged issue_write schema snapshot.
docs/insiders-features.md	Documents flagged issue_write for insiders.
docs/feature-flags.md	Documents flagged issue_write under remote_mcp_issue_fields.

Copilot's findings

• Files reviewed: 21/21 changed files
• Comments generated: 2

[kelsey-myers]

Testing issue_read and issue_write locally with the flag on and off:

issue_read flag on

Input:

{ "method": "get", "owner": "github", "repo": "planning-tracking-demo", "issue_number": 1993 }

Output:

{
  "number": 1993,
  "title": "[test] flag gating sanity - flag ON",
  "state": "open",
  "issue_type": "Task",
  "field_values": [
    { "field": "Priority", "value": "P3" },
    { "field": "Visibility", "value": "Low" }
  ]
}

issue_read flag off

Input:

{ "method": "get", "owner": "github", "repo": "planning-tracking-demo", "issue_number": 1993 }

Output:

{
  "number": 1993,
  "title": "[test] flag gating sanity - flag ON",
  "state": "open",
  "issue_type": "Task"
}

For issue_write, fields are not applied when the flag is off.