agent: Use default prompts from prompt library in system prompt (#28915)

Related to #28490.

- Default prompts from the prompt library are now included as "user
rules" in the system prompt.
- Presence of these user rules is shown at the beginning of the thread
in the UI.
_ Now uses an `Entity<PromptStore>` instead of an `Arc<PromptStore>`.
Motivation for this is emitting a `PromptsUpdatedEvent`.
- Now disallows concurrent reloading of the system prompt. Before this
change it was possible for reloads to race.

Release Notes:

- agent: Added support for including default prompts from the Prompt
Library as "user rules" in the system prompt.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

assets/prompts/assistant_system_prompt.hbs

@@ -144,6 +144,19 @@ In Markdown, hash marks signify headings. For example:
 This example is unacceptable because the path is in the wrong place. The path must be directly after the opening backticks.
 </style>
 
+{{#if has_default_user_rules}}
+The user has specified the following rules that should be applied:
+{{#each default_user_rules}}
+
+{{#if title}}
+Rules title: {{title}}
+{{/if}}
+``````
+{{contents}}
+``````
+{{/each}}
+
+{{/if}}
 The user has opened a project that contains the following root directories/files. Whenever you specify a path in the project, it must be a relative path which begins with one of these root directories/files:
 
 {{#each worktrees}}
@@ -151,7 +164,7 @@ The user has opened a project that contains the following root directories/files
 {{/each}}
 {{#if has_rules}}
 
-There are rules that apply to these root directories:
+There are project rules that apply to these root directories:
 {{#each worktrees}}
 {{#if rules_file}}