feat: support name=value macro arguments with variable tab completion

Macro invocations (.name and .macro name) accept leading name=value
assignments before positional args: assignments set declared variables
directly so earlier variables can keep their defaults, remaining
positionals fill unassigned variables in declaration order, and the
free text after -- is never scanned for assignments. Identifier-shaped
keys that match no declared variable error with the declared list to
catch typos; non-identifier tokens containing = stay positional.
MacroVariable gains an optional description field, and tab completion
after a macro name offers name= candidates showing each variable's
description and default until the assignment prefix ends.

README.md

@@ -39,6 +39,7 @@ Coming from [AIChat](https://github.com/sigoden/aichat)? Follow the [migration g
 * [Macros](https://github.com/Dark-Alex-17/coyote/wiki/Macros): Automate repetitive tasks and workflows with Coyote "scripts" (macros). Macros are Coyote's custom commands: invoke any macro directly by name (e.g. `.review main`), with tab-completion, right alongside the built-in REPL commands.
     * Give a macro a `description` (shown in `.list macros` and completions) and set `isolated: false` to run its steps on the live session, exactly as if you typed them. Note that non-isolated steps are recorded in the session, and mutating steps (`.role`, `.model`, ...) persist after the macro ends — by design. Steps are fail-fast: an error aborts the remaining steps, but completed steps' effects remain. A non-isolated macro step cannot invoke another macro, and a `.exit` step never exits the REPL.
     * Commit project-specific macros to `.coyote/macros/` in your repo — they shadow same-named global macros (opt out with `--no-workspace-macros`).
+    * Pass variables positionally or by name: leading `name=value` args set declared variables directly (letting earlier variables keep their defaults), and remaining args fill the rest in order. Tab completion after a macro name lists each variable with its description and default.
     * Scope which macros are invocable with `enabled_macros` in the global config, a role, an agent, or a session (most specific wins; an empty list disables all macros), and toggle at runtime with `.macro enable|disable <name>`.
 * [RAG](https://github.com/Dark-Alex-17/coyote/wiki/RAG): Retrieval-Augmented Generation for enhanced information retrieval and generation.
 * [Sessions](https://github.com/Dark-Alex-17/coyote/wiki/Sessions): Manage and persist conversational contexts and settings across multiple interactions.