agent: Include grep-related instructions in the prompt only if the tool is available (#29536)

This change updates the system prompt to conditionally include
`grep`-related instructions based on whether the `grep` tool is enabled.

Implementation details:
1. Add a `has_tool` handlebars helper.
2. Pass the `model` to all locations where the prompt is built.
3. Use `{{#if has_tool "grep"}}` in the system prompt to gate
`grep`-specific instructions.

Testing:
- Unit tests for the `hasTool` helper.
- Unit tests to verify that `grep`-related instructions are included /
omitted from the prompt as appropriate.
- Manual agent evaluation:
- Setup: Asked the Agent "List all impls of MyTrait in the project"
using a custom "No tools" profile (all tools disabled).
- Before the change: The Agent attempted to call `grep`, encountered an
error, then realized the tool was unavailable.
- After the change: The Agent immediately asked to enable a search tool.

Note: in principle, `grep`/`read_file` tool descriptions alone might be
enough, but to confirm this we need more evaluation. If it turns out to
be true, we'll be able to remove grep-specific instructions from the
system prompt and undo this change.

Release Notes:

- N/A

assets/prompts/assistant_system_prompt.hbs

@@ -27,13 +27,14 @@ If appropriate, use tool calls to explore the current project, which contains th
 - `{{root_name}}`
 {{/each}}
 
+- Bias towards not asking the user for help if you can find the answer yourself.
 - When providing paths to tools, the path should always begin with a path that starts with a project root directory listed above.
+- Before you read or edit a file, you must first find the full path. DO NOT ever guess a file path!
+{{# if (has_tool 'grep') }}
 - When looking for symbols in the project, prefer the `grep` tool.
 - As you learn about the structure of the project, use that information to scope `grep` searches to targeted subtrees of the project.
-- Bias towards not asking the user for help if you can find the answer yourself.
-{{! TODO: Only mention tools if they are enabled }}
 - The user might specify a partial file path. If you don't know the full path, use `find_path` (not `grep`) before you read the file.
-- Before you read or edit a file, you must first find the full path. DO NOT ever guess a file path!
+{{/if}}
 
 ## Fixing Diagnostics