Model-aware harness extension for pi

# model-harness (pi extension)

A small, **model-aware** harness extension for pi. It evolved from the [Cline `muse-code-harness` plugin](https://github.com/cline/plugins/tree/main/plugins/muse-code-harness) (ported to pi as `muse-code-harness`): instead of one fixed set of conventions, the extension now selects a **profile** from the active model and appends that profile's compact prompt fragment to the run's system prompt.

Profiles are minimal by design — a conservative matcher plus a small, evidence-based prompt. The first two share the original plugin's spirit:

- **`muse-spark`** — the original 8 Muse Spark engineering conventions (preserved verbatim). Also the **default** profile forced in `on` mode when no model matches (preserving the legacy "`/muse-harness on` always injected" semantics).
- **`laguna-s-2.1`** — compact tool-schema discipline for the Laguna S 2.1 / Poolside family.

Footer status shows the **mode**, the **detected profile** (or `none`), and whether a **project override** is active, e.g. `model-harness auto (Muse Spark)` or `model-harness off (none: z-ai/glm-5.2, override)`.

## Install

Already installed globally at `~/.pi/agent/extensions/muse-code-harness/index.ts` on this machine, so pi picks it up automatically in every project. It hot-reloads with `/reload`.

## Per-project overrides

Each profile can be overridden per project via a markdown file at the project's `.pi` config dir:

- `.pi/model-harness/muse-spark.md` — replaces the bundled Muse Spark conventions.
- `.pi/model-harness/laguna-s-2.1.md` — replaces the bundled Laguna S 2.1 fragment.

If a profile's override file is absent, the bundled fragment is used. To **add a profile later**, add one `ProfileDef` entry in `profiles.ts` (a matcher regex, a label, and a prompt fragment) — no other code changes are needed.

### Legacy override (Muse Spark only)

For backward compatibility, `.pi/conventions.md` still replaces the Muse Spark conventions when no `.pi/model-harness/muse-spark.md` exists. (The legacy file is ignored by other profiles.)

Overrides are read lazily on first use and re-read on a new session, a model switch to that profile, or `/model-harness reload`.

## Control

`/model-harness` is the primary command. `/muse-harness` remains as a compatibility alias and does exactly the same thing.

By default the extension runs in **auto mode**: it injects only the prof