docs: explore-minimax memory + ask-minimax usage update

Add /explore-minimax and explore-minimax-run memory files, backfill
skill-ask-minimax / skill-ask-minimax-with-context entries, index them in
MEMORY.md. ask-minimax-usage.md gains the exploration use case and an
explicit no-secrets rule.

Claude-Session: https://claude.ai/code/session_01Ku87DDWwbJkTx6SuV3sQDc

memory/skill-ask-minimax.md

@@ -0,0 +1,15 @@
+# /ask-minimax Skill
+
+**Purpose:** Delegate a file-heavy task (transform, extract, summarise, generate) to a MiniMax M3 sub-session; file contents never enter the parent context, only a concise result. Use when the file payload dwarfs the answer payload.
+
+**Location:** `custom-claude-skills/skills/ask-minimax/`; runs `ask-minimax-run` (small-scripts) headless with `--allowedTools "Read,Write,Edit,Glob,Grep" --permission-mode acceptEdits`, prompt file + result file in `/tmp` with a timestamp suffix.
+
+**Usage guidance:** `claude/ask-minimax-usage.md` (when to use/skip, pass paths not contents, failure handling: retry once, then Claude subagent fallback). Gotchas: `agent-runtimes/memory/gotchas-minimax.md`.
+
+**Related:** [[skill-ask-minimax-with-context]], [[skill-explore-minimax]] (read-only exploration variant), `mm-worktree-task` for planner-backed implementation subagents.
+
+## Gotchas
+
+- Include `Write` in `--allowedTools` whenever the sub-session must create a new file — otherwise MiniMax exits 0 silently with no result.
+- The skill is procedural/sequential, but `ask-minimax-run` itself can be launched 3–4× in parallel via background Bash with unique timestamped files.
+- The sub-session inherits the operator's checked-out branch — do any `git checkout` before invoking.