Fix issue_write silently dropped label errors

[SamMorrowDrums]

Summary

Detect when GitHub accepts an issue create or update request but omits requested labels, and return a precise partial-failure result instead of reporting success.

Why

GitHub can silently ignore labels when the caller lacks AddLabelsToLabelable permission while still creating or updating the issue successfully.

Fixes #3160

What changed

• Compare requested labels with the authoritative label set returned by the existing create/PATCH response, without an additional readback.
• Return an MCP error containing the issue URL plus requested, applied, missing, and unexpected labels; cover create, update, and failed label clearing with focused tests.

MCP impact

• No tool or API changes
• Tool schema or behavior changed — issue_write now reports partial failure when requested labels are not applied.
• New tool added

Prompts tested (tool changes only)

• "Create an issue with the bug and enhancement labels" when GitHub applies only one requested label.
• "Update issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 to add the enhancement label" when GitHub silently leaves the existing labels unchanged.
• "Clear all labels from issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123" when GitHub silently retains an existing label.

Security / limits

• No security or limits impact
• Auth / permissions considered — the error identifies AddLabelsToLabelable as a likely permission boundary without claiming it is the only possible cause.
• Data exposure, filtering, or token/size limits considered

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

• Not needed — the input schema is unchanged.
• Updated (README / docs / examples)

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation correctly detects label mismatches and includes focused regression coverage.

Review tier: Balanced
Findings: None

What changed in this PR

Adds partial-failure detection when issue_write silently fails to apply requested labels.

Changes:

• Compares requested and returned label sets.
• Reports missing/unexpected labels with the issue URL.
• Adds focused create, update, and label-clearing tests.

File	Description
pkg/​github/​issues.go	Adds label reconciliation and partial-failure responses.
pkg/​github/​issues_test.go	Tests unapplied-label scenarios without readback requests.

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