feat(policy): change priority hierarchy to Admin > User > Project > Default

Updates the policy engine to prioritize User policies over Project-specific policies.
This change is a security measure to ensure that users maintain control over their
environment and are not inadvertently compromised by policies defined in a cloned
repository.

Key Changes:
- Swapped Tier 2 (now Project) and Tier 3 (now User).
- Updated documentation to reflect the new hierarchy.
- Updated all built-in policy TOML files with correct tier information.
- Adjusted all tests and integration test expectations to match new priority values.

docs/core/policy-engine.md

@@ -95,8 +95,8 @@ has a designated number that forms the base of the final priority calculation.
 | Tier    | Base | Description                                                                |
 | :------ | :--- | :------------------------------------------------------------------------- |
 | Default | 1    | Built-in policies that ship with the Gemini CLI.                           |
-| User    | 2    | Custom policies defined by the user.                                       |
-| Project | 3    | Policies defined in the current project's configuration directory.         |
+| Project | 2    | Policies defined in the current project's configuration directory.         |
+| User    | 3    | Custom policies defined by the user.                                       |
 | Admin   | 4    | Policies managed by an administrator (e.g., in an enterprise environment). |
 
 Within a TOML policy file, you assign a priority value from **0 to 999**. The
@@ -106,16 +106,16 @@ engine transforms this into a final priority using the following formula:
 
 This system guarantees that:
 
-- Admin policies always override Project, User, and Default policies.
-- Project policies override User and Default policies.
-- User policies always override Default policies.
+- Admin policies always override User, Project, and Default policies.
+- User policies override Project and Default policies.
+- Project policies override Default policies.
 - You can still order rules within a single tier with fine-grained control.
 
 For example:
 
 - A `priority: 50` rule in a Default policy file becomes `1.050`.
-- A `priority: 100` rule in a User policy file becomes `2.100`.
-- A `priority: 10` rule in a Project policy file becomes `3.010`.
+- A `priority: 10` rule in a Project policy file becomes `2.010`.
+- A `priority: 100` rule in a User policy file becomes `3.100`.
 - A `priority: 20` rule in an Admin policy file becomes `4.020`.
 
 ### Approval modes