perf: skip image rebuild when image already exists (fast path for aidc up/claude)

[ai-anant]

Summary

Every aidc up / aidc claude / aidc codex ran docker compose up -d --build, forcing Docker to re-evaluate — and on any cache miss, re-run — the full ~3 GB image build on every start. This is the "every aidc claude or aidc start results in a large ~4GB container" experience.

This PR adds a fast path: the image is built only when it's missing; otherwise compose just starts the container.

Changes

• lib/aidc.sh
  • New aidc::image_exists — resolves the compose image name (docker compose config --images) and checks the local image store.
  • New aidc::compose_up — used by cmd_up and ensure_container_running (the path all tool commands go through). Builds only when the image is missing.
  • aidc rebuild / aidc rescan still call compose directly with --build, so forced rebuilds keep working.
  • AIDC_NO_BUILD=1 (project.env or env) opts out of builds entirely: up starts the existing container and fails fast if the image is missing.
• docs/install.md — documents the fast path and AIDC_NO_BUILD.

Expected impact

• Second and later aidc up / aidc claude on the same project: near-instant (no build evaluation, no downloads).
• Build cache stops accumulating re-downloads of agents/scanners per start.
• First run / after aidc destroy / after aidc rebuild: unchanged (full build).

Test plan

• bash -n lib/aidc.sh
• shellcheck -x lib/aidc.sh — clean
• .github/scripts/bash-compat-check.sh — passes under bash 3.2 (macOS)
• tests/resolve-oauth-token.test.sh — 7/7 pass
• Manual: aidc up twice — second run skips the build (verified via compose output)

Closes #13