feat(copilot): add personal workflow instructions

copilot/instructions/workflow.instructions.md

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+# Personal workflow instructions (Copilot CLI)
+
+Standing habits for how I like implementation work approached in this CLI, independent
+of any one repository. Repo-specific facts and conventions belong in that repo's own
+`.github/copilot-instructions.md` — this file is about *how I like to work*, not about
+any particular codebase.
+
+## Planning
+
+- Use `/plan` for anything beyond a trivial change. Start the plan with branch + worktree
+  setup (follow the target repo's own naming/location conventions if it has them).
+- Structure the plan into one or more phases, each with zero, one, or more commits, and
+  give each planned commit a determined Conventional Commit message up front.
+- Use `/grilling` before committing to a plan whenever there's real ambiguity to resolve —
+  don't skip it just to move faster on non-trivial scope decisions.
+- After I approve a plan, restate the exact in-scope deliverables and keep anything
+  discussion-only (ideas, "what if", tangents) out of scope unless I explicitly promote it.
+
+## Execution
+
+- Prefer sized, reviewable commits over one big end-of-task commit — commit at the end of
+  each phase, including when a fleet of sub-agents executes the phases (each phase agent
+  commits its own work before reporting done).
+- **Model/agent choice per phase is up to the executing agent(s), not pre-assigned by the
+  planner.** Each phase should pick the most cost-effective model/agent capable of doing
+  that phase's work well, and be able to explain the choice if asked. See `model-guide.md`
+  in this same directory for the current model comparison — don't inline that table here,
+  since it changes independently of this workflow.
+- Dispatch parallelizable phases with `/fleet` once the plan is approved.
+- End coding work with a short state summary: changed file count, staged vs. unstaged,
+  branch ahead/behind, and the recommended next action.
+
+## PRs
+
+- Open a PR once the planned work is complete, unless told otherwise.
+- Follow the target repo's own commit/PR conventions (Conventional Commits, merge-vs-squash,
+  etc.) — those live in the repo, not here.