Add update_project_items batch/bulk field-value writes via aliased GraphQL mutations

[veralizeth]

Summary

update_project_items now writes via chunked, aliased updateProjectV2ItemFieldValue / clearProjectV2ItemFieldValue GraphQL mutations (built at runtime with reflect.StructOf) instead of one REST PATCH per item.

Why

Fixes https://github.com/github/planning-tracking/issues/3331 (MCP half only; the CLI half will follow in a separate PR).

Stacking note: this PR is stacked on #2914 (zwick-paginate-project-item-lookup), which extended the issue-number project-item resolver to paginate. This PR's base branch is set to that branch so the diff here only shows the batch-write changes.

What changed

• Extended ResolvedField / projectFieldsCache to also retain the GraphQL field node ID and to support lookup by numeric field ID (in addition to name), amortized once per batch.
• Added node_id as a third item-reference form (alongside item_id and item_owner+item_repo+issue_number), bypassing REST lookup entirely; each item must supply exactly one form.
• Numeric item_id values are deduplicated and resolved to project-item node IDs via bounded-concurrency (5 in flight) REST GETs; the Paginate project item lookup across memberships #2914 paginated issue resolver now also returns the item's node ID.
• updated_field.value is converted to the correct ProjectV2FieldValue member for TEXT, NUMBER, DATE, SINGLE_SELECT, and ITERATION; other data types return a deterministic per-item error directing callers to update_project_item. value: null now routes to the clear mutation instead of sending an empty update.
• Duplicate item+field targets within one call are rejected before any writes, so partitioning update/clear work can't leave the final state ambiguous.
• New pkg/github/projects_batch_mutation.go isolates the reflection: a struct type cached by (kind, chunk size) with positional fields (Item0..ItemN-1), positional GraphQL aliases (item0..), and positional input variables. Per the pinned githubv4.Client.Mutate signature, alias 0's input is always bound to $input; aliases 1+ use $input1, $input2, ... via the variables map (the "first-input workaround"). No owner/id/name/value data is ever embedded in a struct tag.
• Update and clear mutations are executed in separate, sequential chunks of 20 (tool-level cap stays 100 items/call). Per-item results are tri-state (succeeded / failed / unknown): a populated alias is a confirmed success even if the overall response also carried a GraphQL error; an unconfirmed alias after an error is unknown (the pinned client drops errors[].path, so we can't attribute an error to a specific alias). After an ambiguous/transport-level chunk failure or context cancellation, no further chunks are sent — we don't retry or fall back to per-item mutations.
• Removed the stale comment claiming the pinned client couldn't build dynamic aliases (see Query batching and/or dynamic queries. shurcooL/githubv4#17, Use Raw repo resources #70, PR chore: go releaser #71, assert request params and body in pull_request_test #95).

MCP impact

• Tool schema or behavior changed
  update_project_items gains node_id as an item-reference option; its response now includes tri-state status and unknown counts alongside succeeded/failed.

Prompts tested (tool changes only)

• "In octo-org project 1, set the Status field to Done for items 1001 and 1002"
• "Move issue octo/repo#123 to In Progress in project 1, and clear its Estimate field"
• "Update 25 project items' Sprint field in one call" (exercises the 20-item chunk boundary)

Security / limits

• Data exposure, filtering, or token/size limits considered
  Tool-level cap stays at 100 items/call; the internal wire chunk size of 20 is provisional (not benchmarked against the live API) and chosen to leave headroom under typical ~10s gateway/client timeouts for a single aliased-mutation round trip.

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

Lint & tests

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

Docs

• Updated (README / docs / examples)

[Swastiksrijan]

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