Add is_suggestion + rationale to update_issue_assignees

[kelsey-myers]

Summary

Adds is_suggestion + rationale (and confidence) params to the granular update_issue_assignees MCP tool, so agents can propose assignees as pending suggestions rather than applying them directly.

Why

Closes https://github.com/github/plan-track-agentic-toolkit/issues/333

The other granular tools (update_issue_type, update_issue_labels, set_issue_fields) already support intent params behind the issues_granular feature flag. update_issue_assignees was the last granular tool still on the plain string-array form. This brings it to parity, following the same implementation pattern as the labels tool in #2557.

What changed

• Reworked GranularUpdateIssueAssignees from the simple issueUpdateTool wrapper to the object-form pattern: assignees items are now oneOf: [string, object], where the object is { login (required), rationale (≤280 chars), confidence (low/medium/high), is_suggestion }.
• Handler parses string-or-object entries, validates rationale length and confidence enum, and sends either the plain IssueRequest{Assignees:} (no intent) or an object-form body ({ login, rationale, confidence, suggest }) via a raw PATCH — mirroring update_issue_labels.
• Added assigneeWithIntent and assigneesUpdateRequest structs.
• Regenerated the tool snapshot and docs/feature-flags.md.
• Added integration tests: object-form (plain / suggest / rationale+confidence / mixed) and validation errors (rationale too long, missing login, invalid confidence).

MCP impact

• Tool schema or behavior changed — update_issue_assignees now accepts an object form per assignee with rationale / confidence / is_suggestion. The plain string-array form is unchanged and still serializes identically, so it's backwards compatible.

Prompts tested (tool changes only)

Tested locally against dotcom via a local build (bin/github-mcp-server-local, --features issues_granular):

• Suggest an assignee with rationale + high confidence → issue returned, assignee not applied (pending suggestion created), confirmed via the API.
• Invalid confidence value → tool error confidence must be one of: low, medium, high.
• Malformed entry (123) → tool error each assignee must be a string or an object….
• Empty assignees array → clean no-op.
• Non-assignable login with is_suggestion: true → dotcom 422 surfaced as a clean tool error.

Security / limits

• No security or limits impact — same auth/scopes (repo) and endpoint as before; only the request-body shape gained optional intent fields. Rationale is capped at 280 chars.

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) — regenerated via script/generate-docs.

[Copilot]

Pull request overview

This PR updates the issues_granular feature-flagged MCP tool update_issue_assignees to support an “intent” object form per assignee (including is_suggestion, rationale, and confidence), bringing it in line with the object-form patterns already used by other granular issue update tools.

Changes:

• Replaces the previous issueUpdateTool wrapper for update_issue_assignees with a custom handler that accepts assignees items as string | { login, rationale?, confidence?, is_suggestion? }.
• Adds request-shaping logic to preserve the legacy wire format when no intent fields are used, and switches to an object-form PATCH body when intent fields are present.
• Updates the tool snapshot and adds unit tests for object-form serialization and validation errors.

Show a summary per file

File	Description
pkg/github/issues_granular.go	Reworks update_issue_assignees schema + handler to support per-assignee intent fields and object-form requests.
pkg/github/granular_tools_test.go	Adds tests for the new object-form behavior and validation paths.
pkg/github/toolsnaps/update_issue_assignees.snap	Updates the snapshot to reflect the new assignees oneOf schema.
docs/feature-flags.md	Regenerates feature-flag tool parameter docs for update_issue_assignees.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Low