Docs: marketplace, and clean up new-code warnings/lints

CLAUDE.md gets a Marketplace subsection under Key Conventions (the sync
script is app-embedded and re-uploaded on every sync, never baked into
container/ — pre-flight F9) and the Settings export/import section now
covers marketplace account tokens traveling in ExportedSecrets and the
import preview's warning on global hook and plugin installs.
HOW-TO-USE.md gets a Marketplace section (placed after Shared Claude
Authentication) with its Table of Contents entry (pre-flight N13). The
spec doc's stale keychain service name, gh-login flags and
upload_bytes_to_container signature are amended to match the shipped
code (pre-flight N10).

Also fixes the new marketplace code's remaining build/clippy warnings:
BTreeMap/Sha256/Digest imports in tree.rs gated behind #[cfg(test)]
(their only uses are on MemTree, already test-only), the unused
`pub use marketplace::*` glob re-export dropped from models/mod.rs,
gh_login::strip_ansi marked #[cfg(test)] (production streams through
AnsiStripper instead), and four clippy lints in marketplace test code
(double_ended_iterator_last, cloned_ref_to_slice_refs x2,
single_match). Flushes the unresolved getMarketplaceSyncReport promise
in MarketplaceSection.test.tsx's "opens the Marketplace filtered to
this project" test to remove its act() warning.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src-tauri/src/marketplace/git.rs

@@ -91,7 +91,7 @@ pub fn classify_fetch_error(chain: &str) -> FetchError {
         let cause = chain
             .lines()
             .filter_map(|l| l.trim_start().strip_prefix("└─"))
-            .last()
+            .next_back()
             .unwrap_or(chain);
         return FetchError::Network(first_line(cause));
     }
@@ -600,7 +600,7 @@ mod tests {
         want.sort();
         assert_eq!(pins(&repo), want);
 
-        set_pins(&repo, &[b.clone()]).unwrap();
+        set_pins(&repo, std::slice::from_ref(&b)).unwrap();
         assert_eq!(pins(&repo), vec![format!("{}{}", PIN_PREFIX, b)]);
     }