fix: apply cargo clippy auto-fixes across workspace

Summary

Ran cargo clippy --fix and cargo clippy across the workspace to resolve all auto-fixable clippy warnings. Changes are purely mechanical/cosmetic with no behavioral changes:

• Remove unused import: use tauri::Manager in desktop/src-tauri/src/lib.rs
• Remove redundant conversion: &distinct_id.to_string() → distinct_id (already a &str)
• Collapse nested if-let into let-chains: Several instances across tiptap, fs-db, and listener where nested if/if let blocks are flattened using Rust's let-chain syntax (if cond && let ... && ...)
• Remove unnecessary lifetime annotations: group_by_session_id<'a, T, F> → group_by_session_id<T, F> in two migration files (elision handles it)
• Exclude plugins/cli2 from workspace: This directory only contains node_modules with no Cargo.toml, causing workspace resolution failures

Note: Several packages (tools, control-tauri, notch, am2, tcc, notification-macos) could not be checked because they depend on macOS-only system libraries (swift_rs, libspa, tauri-nspanel) unavailable on Linux.

Review & Testing Checklist for Human

• Verify let-chain formatting: The auto-fixer produced unusual indentation on the collapsed let-chains (e.g., from_ast.rs, macro.rs). Confirm the style is acceptable or run cargo fmt to normalize.
• Verify let-chain semantics in from_ast.rs: The unescape_markdown function's control flow is correct after collapsing — the result.push(c) fallthrough and continue behavior should be identical to the original nested version.
• Confirm plugins/cli2 exclusion is intentional: The directory has no Cargo.toml (removed in commit 9a079475b). Adding it to exclude fixes workspace resolution; verify this is the desired fix vs. removing the directory entirely.

Notes

• Requested by: unknown ()
• Link to Devin run: https://app.devin.ai/sessions/fac7891ac9c14bb88ef6564fe937fbaf