feat(adapters): adapter contract, dispatch, pi + mock adapters (#16)

- adapters/README.md: the harness boundary contract (env in, response on
  stdout, diagnostics stderr, exit 0 success)
- adapters/pi: extracted current invocation unchanged
- adapters/mock: deterministic MOSAIC_MOCK_RESPONSE echo (test-only)
- run-agent.sh: name-validated dispatch to adapters/<name>/adapter.sh
- config: optional execution.adapter (pi|mock), default pi, configVersion
  stays 1 — existing configs remain valid; selection authority is the
  config file (load_config exports it)
- compose: MOSAIC_ADAPTER / MOSAIC_MOCK_RESPONSE passthrough; Containerfile
  installs adapters read-only; RELEASE -> 0.0.5

Verified: hello unchanged; mock verbatim via config; unknown adapter and
path-traversal names refused in-container; invalid adapter exits 2.

Closes #16

adapters/README.md

@@ -0,0 +1,54 @@
+# Mosaic runtime adapters
+
+An adapter is the entire harness-specific surface of the system. Everything
+upstream of an adapter — configuration, contracts, missions, tasks, run
+records — is harness-agnostic; everything inside an adapter may assume one
+specific agent runtime.
+
+## Contract
+
+An adapter lives at:
+
+```text
+/opt/mosaic/adapters/<name>/adapter.sh
+```
+
+and must be executable. The dispatcher (`/opt/mosaic/src/run-agent.sh`)
+selects it via `MOSAIC_ADAPTER` (default: `pi`) and execs it after the
+system prompt has been generated.
+
+**Inputs (environment):**
+
+| Variable | Meaning |
+|---|---|
+| `MOSAIC_SYSTEM_PROMPT_FILE` | Absolute path to the generated system prompt (contracts + optional mission section). Read it; do not modify it. |
+| `MOSAIC_REQUEST` | The exact user request text (may contain newlines). |
+| `MOSAIC_PROVIDER` | Configured provider name. |
+| `MOSAIC_MODEL` | Configured model id. |
+
+Optional, adapter-specific (documented per adapter):
+
+| Variable | Meaning |
+|---|---|
+| `MOSAIC_MOCK_RESPONSE` | mock only: the verbatim response to emit |
+
+**Outputs:**
+
+- `stdout`: the model response text — the only channel the orchestrator captures
+- `stderr`: diagnostics (never credentials)
+- exit `0`: success; nonzero: failure
+
+## Rules
+
+1. Adapters print ONLY the response on stdout. Status lines go to stderr.
+2. Adapters never read configuration files; the resolved settings arrive via environment.
+3. Adapters never write outside `/var/lib/mosaic`.
+4. Adding an adapter requires: a new directory, the contract implementation, and
+   adding the name to the allowlist in `scripts/mosaic-config.mjs`.
+
+## Included adapters
+
+- `pi` — the pinned `@earendil-works/pi-coding-agent` CLI in noninteractive
+  print mode (`-p`), ambient discovery disabled, stdin detached.
+- `mock` — deterministic echo of `MOSAIC_MOCK_RESPONSE`. Test-only: never use
+  it where a real model response is required.