feat: add compaction worker extension

compaction-worker/LICENSE

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+MIT License
+
+Copyright (c) 2026 Will Porcellini
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

compaction-worker/README.md

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+# Pi Compaction Worker
+
+Opt-in Pi extension for model-aware custom compaction.
+
+It provides an extension-first slice for issue #595:
+
+- resolves per-model/profile compaction budgets;
+- starts a side summary job at a configurable prepare threshold;
+- triggers compaction at a later threshold;
+- validates prepared summaries against the current session/branch before use;
+- falls back to live custom compaction, then Pi default compaction, on any failure.
+
+The extension is disabled by default.
+
+## Configuration
+
+Add settings under `compaction-worker` in `.pi/settings.json` or `~/.pi/agent/settings.json`:
+
+```jsonc
+{
+  "compaction-worker": {
+    "enabled": true,
+    "prepareAtPercent": 60,
+    "triggerAtPercent": 75,
+    "reserveTokens": 16384,
+    "keepRecentTokens": 20000,
+    "summaryModels": ["google/gemini-2.5-flash"],
+    "profiles": {
+      "opus": {
+        "match": "anthropic/claude-opus-*",
+        "prepareAtPercent": 55,
+        "triggerAtPercent": 70,
+        "keepRecentTokens": 100000,
+      },
+    },
+  },
+}
+```
+
+Use `/compaction-worker-status` to inspect the active model, matched profile, thresholds, and prepared-summary state.
+
+Profile matching is deterministic: exact model matches win over globs, more specific globs win over broader globs, and declaration order is only used as the final tie-breaker.
+
+Additional guardrail knobs are available when you need to coordinate with Pi's built-in compaction threshold:
+
+- `builtinReserveTokens` (default `16384`) mirrors the host built-in reserve used to detect the built-in threshold.
+- `builtinSkipMarginPercent` (default `0`) can reserve a small percentage band before that built-in threshold where this worker stands down and lets Pi's built-in compaction path win. Keep this at `0` unless you explicitly want that handoff band.
+
+## Boundaries
+
+This is an extension-side proactive policy. Pi core still owns built-in auto-compaction and overflow recovery thresholds. If a prepared summary is stale, divergent, expired, or incompatible with the current model/profile, the extension generates a live summary or falls back to Pi default compaction.

compaction-worker/config.ts

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+import * as fs from "node:fs";
+import * as os from "node:os";
+import { join } from "node:path";
+import {
+  CONFIG_ENV,
+  SETTINGS_KEY,
+  resolveBaseConfig,
+  type RawCompactionWorkerConfig,
+  type ResolvedBaseConfig,
+} from "./helpers.js";
+
+export interface LoadConfigOptions {
+  cwd?: string;
+  agentDir?: string;
+  env?: NodeJS.ProcessEnv;
+}
+
+function parseJsonFile(filePath: string): unknown | null {
+  try {
+    return JSON.parse(fs.readFileSync(filePath, "utf8")) as unknown;
+  } catch (error) {
+    const message = error instanceof Error ? error.message : String(error);
+    console.error(`[${SETTINGS_KEY}] Failed to parse config ${filePath}: ${message}`);
+    return null;
+  }
+}
+
+function readSettingsConfig(
+  settingsPath: string,
+): { path: string; raw: RawCompactionWorkerConfig } | null {
+  if (!fs.existsSync(settingsPath)) return null;
+  const parsed = parseJsonFile(settingsPath);
+  if (!parsed || typeof parsed !== "object") return null;
+  const raw = (parsed as Record<string, unknown>)[SETTINGS_KEY];
+  if (!raw || typeof raw !== "object") return null;
+  return {
+    path: `${settingsPath}#${SETTINGS_KEY}`,
+    raw: raw as RawCompactionWorkerConfig,
+  };
+}
+
+export function loadConfig(options: LoadConfigOptions = {}): ResolvedBaseConfig {
+  const cwd = options.cwd ?? process.cwd();
+  const agentDir = options.agentDir ?? join(os.homedir(), ".pi", "agent");
+  const env = options.env ?? process.env;
+
+  const explicitSettingsPath = env[CONFIG_ENV];
+  if (explicitSettingsPath) {
+    const explicit = readSettingsConfig(explicitSettingsPath);
+    if (explicit) return resolveBaseConfig(explicit.raw, explicit.path);
+  }
+
+  const projectSettings = readSettingsConfig(join(cwd, ".pi", "settings.json"));
+  if (projectSettings) return resolveBaseConfig(projectSettings.raw, projectSettings.path);
+
+  const globalSettings = readSettingsConfig(join(agentDir, "settings.json"));
+  if (globalSettings) return resolveBaseConfig(globalSettings.raw, globalSettings.path);
+
+  return resolveBaseConfig(null, null);
+}

compaction-worker/eslint.config.mjs

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+import rootConfig from "../eslint.config.mjs";
+
+export default [...rootConfig];