Add rationale and is_suggestion to granular assignee tool

[alondahari]

Summary

Updates GranularUpdateIssueAssignees to support polymorphic input — each assignee can be a plain string (login) or an object with login, rationale, confidence, and is_suggestion fields. This follows the exact same pattern already established by GranularUpdateIssueLabels.

Changes

• assigneeWithIntent struct — mirrors labelWithIntent with login, rationale, confidence, and suggest fields
• assigneesUpdateRequest struct — mirrors labelsUpdateRequest for custom request bodies
• Polymorphic schema — assignees items accept oneOf: [string, object] with the object form supporting login, rationale, confidence, and is_suggestion
• Custom request path — when any assignee has rationale/confidence/is_suggestion, builds object-form JSON via client.NewRequest + client.Do (bypassing the go-github IssueRequest); otherwise preserves the standard string-array wire format
• Backward compatible — plain string arrays continue to work identically

Tests

• TestGranularUpdateIssueAssignees — plain strings, object-without-rationale fallback, mixed, []string typed array
• TestGranularUpdateIssueAssigneesSuggest — suggestion flag alone, with rationale, mixed
• TestGranularUpdateIssueAssigneesInvalidRationale — too-long rationale, missing login
• TestGranularUpdateIssueAssigneesConfidence — confidence triggers object form, normalization, invalid value

Fixes github/plan-track-agentic-toolkit#333

[Copilot]

Pull request overview

This PR extends the granular update_issue_assignees tool to accept polymorphic assignee inputs (either plain logins or structured objects carrying rationale/confidence/suggestion intent), matching the established “with intent” pattern already used by other granular issue update tools.

Changes:

• Updated update_issue_assignees input schema to accept oneOf assignee items: string or { login, rationale, confidence, is_suggestion }.
• Implemented object-form request serialization when any assignee includes intent metadata, while preserving the legacy string-array wire format when not needed.
• Added expanded unit test coverage for mixed inputs, suggestions, rationale validation, and confidence normalization; updated the corresponding toolsnap and generated docs.

Show a summary per file

File	Description
pkg/github/issues_granular.go	Adds polymorphic assignee parsing + conditional request body shaping for intent metadata.
pkg/github/granular_tools_test.go	Adds/expands tests covering string/object assignees, suggestions, rationale validation, and confidence normalization.
pkg/github/toolsnaps/update_issue_assignees.snap	Updates the snapshot to reflect the new schema/description for update_issue_assignees.
docs/insiders-features.md	Regenerates docs with updated scope phrasing for ui_get.
docs/feature-flags.md	Regenerates docs reflecting the updated assignees tool schema and scope phrasing for ui_get.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2