Replace update_project_items REST loop with chunked aliased GraphQL mutations

Rewrite update_project_items to resolve the project, every distinct
field, and every item once up front, then write via chunked, aliased
updateProjectV2ItemFieldValue / clearProjectV2ItemFieldValue GraphQL
mutations built at runtime with reflect.StructOf, instead of one REST
PATCH per item.

- Extend ResolvedField/projectFieldsCache with GraphQL node IDs and
  numeric-ID lookup, alongside existing name-based lookup.
- Add node_id as a third item-reference form (alongside item_id and
  item_owner/item_repo/issue_number), bypassing REST lookup entirely.
- Deduplicate numeric item_id lookups with bounded-concurrency REST
  GETs; extend the #2914 paginated issue resolver to also return the
  item's node ID.
- Convert updated_field.value to the correct ProjectV2FieldValue member
  for TEXT, NUMBER, DATE, SINGLE_SELECT, and ITERATION; route null
  values to the clear mutation instead of an empty update.
- Reject duplicate item+field targets before any writes.
- Isolate the reflection mechanics in projects_batch_mutation.go: a
  cached-by-(kind,size) reflected struct type with positional aliases
  (item0..itemN-1) and positional input variables (, ,
  ...), matching the first-input workaround required by the pinned
  githubv4.Client.Mutate signature.
- Execute updates then clears in sequential chunks of 20 (provisional,
  documented headroom rationale), tracking tri-state per-item results
  (succeeded/failed/unknown); abort remaining chunks after an ambiguous
  transport-level failure or context cancellation without retrying.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: d7dc302d-e6f2-41e9-a2c8-ed598de47067

Author: zwick

README.md

@@ -1101,7 +1101,7 @@ The following sets of tools are available:
   - `item_owner`: The owner (user or organization) of the repository containing the issue or pull request. Required for 'add_project_item' method. Also accepted by 'update_project_item' when resolving the item by issue number. (string, optional)
   - `item_repo`: The name of the repository containing the issue or pull request. Required for 'add_project_item' method. Also accepted by 'update_project_item' when resolving the item by issue number. (string, optional)
   - `item_type`: The item's type, either issue or pull_request. Required for 'add_project_item' method. (string, optional)
-  - `items`: The set of items to update. Required for 'update_project_items'. Each entry references either an existing item by 'item_id', or by (item_owner + item_repo + issue_number). Each entry may optionally include its own 'updated_field'; if omitted, the top-level 'updated_field' is applied. Limit: 100 items per call. (object[], optional)
+  - `items`: The set of items to update. Required for 'update_project_items'. Each entry references an existing item by exactly one of: 'node_id' (the item's GraphQL node ID, e.g. as returned by 'list_project_items' or 'add_project_item'), 'item_id' (the numeric project item ID), or (item_owner + item_repo + issue_number). Each entry may optionally include its own 'updated_field'; if omitted, the top-level 'updated_field' is applied. Limit: 100 items per call. (object[], optional)
   - `iteration_duration`: Duration in days for iterations of the field (e.g. 7 for weekly, 14 for bi-weekly). Required for 'create_iteration_field' method. (number, optional)
   - `iterations`: Custom iterations for 'create_iteration_field' method. Only set this when you need iterations with varying durations, breaks between them, or specific titles. Otherwise omit it: GitHub auto-creates three iterations of 'iteration_duration' days starting on 'start_date', which is the right choice for most cases. (object[], optional)
   - `method`: The method to execute (string, required)

pkg/github/__toolsnaps__/projects_write.snap

@@ -41,7 +41,7 @@
         "type": "string"
       },
       "items": {
-        "description": "The set of items to update. Required for 'update_project_items'. Each entry references either an existing item by 'item_id', or by (item_owner + item_repo + issue_number). Each entry may optionally include its own 'updated_field'; if omitted, the top-level 'updated_field' is applied. Limit: 100 items per call.",
+        "description": "The set of items to update. Required for 'update_project_items'. Each entry references an existing item by exactly one of: 'node_id' (the item's GraphQL node ID, e.g. as returned by 'list_project_items' or 'add_project_item'), 'item_id' (the numeric project item ID), or (item_owner + item_repo + issue_number). Each entry may optionally include its own 'updated_field'; if omitted, the top-level 'updated_field' is applied. Limit: 100 items per call.",
         "items": {
           "additionalProperties": false,
           "properties": {
@@ -50,7 +50,7 @@
               "type": "number"
             },
             "item_id": {
-              "description": "The project item ID. Provide either item_id, or (item_owner + item_repo + issue_number).",
+              "description": "The project item ID. Provide exactly one of node_id, item_id, or (item_owner + item_repo + issue_number).",
               "type": "number"
             },
             "item_owner": {
@@ -61,6 +61,10 @@
               "description": "Repository containing the issue or pull request. Combine with item_owner and issue_number to resolve the item.",
               "type": "string"
             },
+            "node_id": {
+              "description": "The project item's GraphQL node ID. Provide exactly one of node_id, item_id, or (item_owner + item_repo + issue_number).",
+              "type": "string"
+            },
             "updated_field": {
               "description": "Per-item field update. Overrides the top-level 'updated_field'. Same shape as the top-level 'updated_field'.",
               "type": "object"