fix: improve issue-triage prompt robustness to prevent no-output failures (#10504)

Agentic Triage workflow ([run
24996553319](https://github.com/microsoft/typespec/actions/runs/24996553319))
produced no safe outputs despite the agent running for ~4 minutes. This
is the second failure after PR #10502 fixed tool name references — the
agent still silently failed to call any safe output tool.

Analysis of the generated prompt and MCP gateway logs revealed three
issues:

- **Missing `get_file_contents` tool reference**: The prompt said "read
the file `eng/common/config/labels.ts`" without specifying the MCP tool
name or parameters. Now explicitly references `get_file_contents` with
`owner`/`repo`.
- **Unconditional `web-fetch` reference**: The prompt unconditionally
instructed the agent to "Use `web-fetch` to verify the playground link",
but only `safeoutputs` and `github` MCP servers were registered —
`web-fetch` was not available. Changed to conditional: "If the
`web_fetch` tool is available, use it to verify..."
- **No fallback instruction**: Added an explicit `## Important` section
reinforcing the requirement to call at least one safe output tool
(`add_comment`, `add_labels`, or `noop`) before finishing, with guidance
to call `noop` on errors.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: timotheeguerin <1031227+timotheeguerin@users.noreply.github.com>

Author: Copilot

.github/workflows/issue-triage.md

@@ -48,7 +48,7 @@ You're a triage assistant for the TypeSpec GitHub repository. Your task is to an
 
 ## Available Labels
 
-To find the available labels for this repository, read the file `eng/common/config/labels.ts` in the repository. This file contains all the label definitions organized by category:
+To find the available labels for this repository, use the `get_file_contents` tool to read the file `eng/common/config/labels.ts` (owner: `microsoft`, repo: `typespec`). This file contains all the label definitions organized by category:
 
 - **Area labels** (`AreaLabels`): Used to categorize which area of the codebase the issue belongs to (e.g. `compiler:core`, `emitter:openapi3`, `ide`, etc.). Always try to assign the most specific area label that matches.
 - **Issue kind labels** (`issue_kinds`): Used to classify the type of issue (`bug`, `feature`, `docs`).
@@ -104,7 +104,7 @@ Only use labels defined in that file.
    - Do NOT change the semantics of the reproduction
 
    **Construct a playground link**: If the issue has reproduction TypeSpec code but no playground link, construct one using this URL format:
-   `https://typespec.io/playground?c={encoded}` where `{encoded}` is the repro code URL-encoded (percent-encoded). Use `web-fetch` to verify the playground link works by fetching it.
+   `https://typespec.io/playground?c={encoded}` where `{encoded}` is the repro code URL-encoded (percent-encoded). If the `web_fetch` tool is available, use it to verify the playground link works by fetching it.
 
    Skip this step entirely for feature requests and documentation issues.
 
@@ -143,3 +143,7 @@ Only use labels defined in that file.
    - Skip the reproduction block entirely for feature requests and docs issues
    - Skip "Suggested next steps" if there's nothing actionable to suggest
    - Do NOT include any text about agentic workflows, how to install them, or any self-promotional content. Do NOT mention AI, bots, or agents.
+
+## Important
+
+You MUST call at least one safe output tool (`add_comment`, `add_labels`, or `noop`) before finishing. If you encounter errors with any tool or cannot complete the triage, call `noop` with a message describing what went wrong.