fix: use open_in_memory() for DB init fallback to avoid PermissionDenied panic

[txf0096]

Problem

On systems where /tmp is permission-restricted, git-ai checkpoint panics:

called `Result::unwrap()` on an `Err` value: Os { code: 1, kind: PermissionDenied, message: "Operation not permitted" }

Backtrace:

 8: core::result::unwrap_failed
 9: git_ai::commands::git_ai_handlers::handle_checkpoint
10: git_ai::main

Root Cause

When InternalDatabase::new() fails (e.g. due to SQLITE_BUSY from concurrent processes), both global() functions fall back to a dummy connection via std::env::temp_dir(). On systems where /tmp is noexec or permission-restricted, Connection::open(&temp_path).expect(...) panics — turning a graceful degradation into a crash.

// src/authorship/internal_db.rs and src/metrics/db.rs
let temp_path = std::env::temp_dir().join("git-ai-db-failed");
let conn = Connection::open(&temp_path).expect("Failed to create temp DB"); // panics on PermissionDenied

Additionally, the fixed path git-ai-db-failed is shared across all processes — if multiple processes hit the fallback simultaneously, they contend on the same file, potentially causing the same lock error again.

The comment already describes the intent:

// Create a dummy connection that will fail on any operation
// This allows the program to continue even if DB init fails

Fix

Replace the fallback with an in-memory connection in both src/authorship/internal_db.rs and src/metrics/db.rs:

let conn = Connection::open_in_memory()
    .expect("Failed to create in-memory fallback DB");
Mutex::new(InternalDatabase {
    conn,
    _db_path: PathBuf::new(),
})

open_in_memory() matches the original intent exactly — all SQL operations still fail (no schema initialized), the program continues without panic, no filesystem dependency, and each process gets its own independent connection.

Impact

• git-ai stats is not affected (reads from git notes, not internal_db)
• Core AI attribution tracking is not affected (DB writes are non-fatal)
• Affected when fallback triggers: git-ai search, git-ai prompts, git-ai show-prompt, git-ai share — return errors or empty results (same as before, just without the panic)

Fixes #645

[git-ai-cloud-dev]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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