feat(cli): implement all template functions, auto-install local tools, and config fixes

[appcypher]

PR Title: feat(cli): implement all template functions, auto-install local tools, and config fixes

Summary

• Implement all 13 MCPBX spec template functions with a recursive descent expression parser, replacing the previous flat if/else chain that only supported base64 and basicAuth
• Add auto-install for local path tools so that tool info/call/run/config set on a local path automatically links the tool, enabling config to be stored by name
• Fix config unset to work on stored config names even when the tool is not installed
• Fix config re-prompting so optional fields are not re-prompted on subsequent runs
• Add single-letter subcommand aliases and host add validation

These changes make local tool workflows seamless: users can point at a local directory and the CLI handles installation, config storage, and cleanup without manual install steps.

Changes

lib/vars.rs (+684/-68) - Complete rewrite of template variable substitution:

• Replace flat if/else dispatch with recursive descent parser (eval_expr, resolve_var, split_args, eval_func)
• Implement all 13 MCPBX template functions: base64, base64url, urlEncode, hex, concat, lower, upper, trim, default, basicAuth, bearer, timestamp, uuid, jsonEncode
• Support nested function calls like ${base64(concat(user_config.user, ':', user_config.pass))}
• Fix basicAuth to include "Basic " prefix per spec
• Add 37 unit tests covering all functions, nesting, edge cases, and error handling

lib/handlers/tool/registry.rs (+109) - New local tool linking functions:

• Add LinkResult enum (Linked, AlreadyLinked, Conflict)
• Add link_local_tool for symlink creation with conflict detection
• Add link_local_tool_force for overwriting existing links
• Add cross-platform create_symlink helper

lib/handlers/tool/common.rs (+99/-6) - Auto-install integration:

• Add auto_install_local_tool shared helper with conflict prompt (Overwrite? [y/N])
• Call auto-install in prepare_tool when is_installed = false, covering tool info/call/run

lib/handlers/tool/config_cmd.rs (+64/-17) - Config command fixes:

• Call auto-install in config_set for local path tools
• Add resolve_tool_for_config helper that falls back to PluginRef lookup when tool resolution fails
• Update unset_tool_keys and unset_tool_all_keys to use the fallback resolver
• Add tool_config_exists function for checking config file existence on disk

lib/handlers/tool/call.rs (+46/-14) - Config prompt improvements:

• Return has_saved_config from parse_user_config based on actual file existence
• Update prompt_missing_user_config to only prompt required fields without defaults on subsequent runs

lib/handlers/tool/host_cmd.rs (+14) - Host add validation:

• Validate user-specified tools are installed via FilePluginResolver::resolve_tool
• Error with hint to run tool install if not found

lib/commands.rs (+14/-7) - Subcommand aliases:

• Add aliases for config subcommands: s (set), g (get), l (list), u (unset)
• Add aliases for host subcommands: a (add), r (remove), l (list), p (preview)

Test Plan

• Run cargo build to verify compilation
• Run cargo test to verify all 140 tests pass (including 37 new template function tests)
• Test template functions: tool info external/mongodb with a manifest using ${default(user_config.connection_string, '')} should resolve without "Undefined variable" errors
• Test auto-install: tool info ./path/to/local/tool should print "Linked name from path" on first use
• Test auto-install conflict: manually install a tool, then run tool info ./different/path for same name -- should prompt "Overwrite? [y/N]"
• Test config unset fallback: tool config set ./local-tool key=value, then uninstall the tool, then tool config unset local-tool --all should succeed
• Test config re-prompting: run tool info external/mongodb twice -- second run should not re-prompt for optional fields
• Test subcommand aliases: tool config s <tool>, tool host a claude-desktop
• Test host add validation: tool host add claude-desktop nonexistent-tool should error with install hint