Address code review comments from PR #1518

This commit addresses the unresolved review comments from the merged PR
that added the josh CLI tool:

## Error Handling Improvements
- Add anyhow and thiserror to workspace dependencies
- Replace Box<dyn std::error::Error> with anyhow::Result throughout josh-cli
- Use anyhow::Context for better error messages with context

## josh-core/src/changes.rs Improvements
- Use into_iter() instead of iter() where changes vec is consumed
- Replace manual indexing with .windows(2) for cleaner iteration
- Use iterator chaining with base_tree for tree collection
- Use HashSet::insert() which returns false for duplicates
- Improve variable naming: old -> base_oid, v -> push_refs
- Change ref_with_options parameter from String to &str for consistency
- Replace if let with match for better pattern matching
- Use filter_map instead of filter + map

## josh-cli/src/bin/josh.rs Improvements
- Remove all std::env::set_current_dir() calls (hacky global state)
- Create internal helper functions that accept repo_path parameter:
  - handle_fetch_internal(args, repo_path)
  - handle_remote_add_internal(args, repo_path)
  - handle_filter_internal(args, repo_path, print_messages)
  - apply_josh_filtering(repo_path, filter, remote_name)
- Remove commented out code
- Update all error handling to use anyhow::Context
- Fix build_to_push call to pass &remote_ref instead of remote_ref

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Vlad Ivanov <vlad-ivanov-name@users.noreply.github.com>

Author: github-actions[bot]

Cargo.lock

@@ -20,6 +20,7 @@ opt-level = 3
 codegen-units = 1
 
 [workspace.dependencies]
+anyhow = "1.0"
 base64 = "0.22.1"
 defer = "0.2.1"
 env_logger = "0.11.5"

Cargo.toml

@@ -13,6 +13,7 @@ version = "22.4.15"
 josh = { path = "../josh-core" }
 josh-graphql = { path = "../josh-graphql" }
 josh-templates = { path = "../josh-templates" }
+anyhow = { workspace = true }
 env_logger = { workspace = true }
 log = { workspace = true }
 serde_json = { workspace = true }