Add project rule context diagnostics

[szgupta]

Description

Adds warn-level diagnostics around project rule context propagation to help debug inconsistent active_rule_files population.

Specifically, this logs when:

• project rule scans cannot read metadata, are pointed at a non-directory, fail to build the file tree, exceed the scan file limit, or cannot read an individual rule file
• project rule lookup finds indexed rules but no active rule files for the current path
• pending AI context has project rules with no active rules or empty rule content
• API conversion produces empty or mismatched active_rule_files
• restored conversation context has project rules with missing active_rule_files

The scan now continues after a single unreadable rule file instead of aborting the rest of the candidate rule files. The logs include paths and counts, but not rule file contents.

Oz conversation: https://staging.warp.dev/conversation/ef46e7dd-4e1f-4c9f-aa80-a743789d309e

Linked Issue

N/A

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Screenshots / Videos

N/A — logging-only change.

Testing

• cargo fmt
• cargo check -p warp --lib --features local_fs
• cargo clippy --workspace --all-targets --all-features --tests -- -D warnings
• Re-ran cargo fmt and cargo clippy --workspace --all-targets --all-features --tests -- -D warnings after merging origin/master.
• After adding scan failure diagnostics: cargo fmt, cargo check -p warp --lib --features local_fs, and cargo clippy --workspace --all-targets --all-features --tests -- -D warnings.

No new tests were added because this is diagnostic logging only and does not change project-rule behavior, aside from continuing a scan after a single unreadable candidate rule file.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Co-Authored-By: Oz oz-agent@warp.dev

[szgupta]

I will likely revert this one I've found the issue