feat: split image into shared aidc-base + thin per-project layer

[ai-anant]

Summary

Every project was building the entire ~3 GB image from scratch: base OS + apt tools + uv/Python + all five coding agents + all scanners. Measured floor: 3.22 GB with zero toolchains. Ten projects = ~32 GB of images, ~30 GB of it identical content, and every new project paid a 10–20 minute build.

This PR splits the image into:

1. templates/devcontainer/Dockerfile.base.tmpl (new) — everything common: base OS, apt, uv/Python, agents, scanners, pmg wiring, clipboard bridge, bootstrap-claude. Built once per content hash as aidc-base:<hash> and shared by every project.
2. templates/devcontainer/Dockerfile.tmpl — now FROM aidc-base:<hash> and only adds the per-project toolchains, opt-in security tools, and project-setup.sh.

This is the "one copy of Python serves 10,000 sandboxes" idea from https://x.com/AlexJonesax/status/2083617647342600591 — projects boot from a shared, pre-verified base instead of rebuilding the world.

Changes

• lib/aidc.sh
  • aidc::ensure_base_image — builds the hashed base image when missing, called from cmd_up, cmd_rebuild, cmd_rescan, and ensure_container_running before the compose build; exports AIDC_BASE_IMAGE so the project Dockerfile's FROM resolves.
  • aidc::base_image_tag — content hash of Dockerfile.base (+ AIDC_AGENTS for forward-compat with the opt-in agents PR), so template edits flow into a new tag automatically.
  • AIDC_BASE_IMAGE in project.env pins a custom base.
  • .devcontainer/Dockerfile.base added to AIDC_MANAGED_PATHS and scaffolded by aidc init.
• templates/devcontainer/compose.yaml.tmpl — passes AIDC_BASE_IMAGE as a build arg.
• .github/workflows/aidc-e2e.yml — asserts the new scaffold file.
• README.md — documents the shared base.

Verification (measured)

• Base image builds (aidc-base:4136caa40095, 25 layers) and the thin project image builds FROM it (30 layers).
• Only 5 of the project image's 30 layers are unique (toolchain delta, ~0–100 MB) — the other 25 (≈3.2 GB) are shared on disk with the base.
• bash -n, shellcheck -x, bash-3.2 compat, tests/resolve-oauth-token.test.sh (7/7) — clean.

Expected impact

• Fresh aidc up on a new project: builds only the toolchain delta (minutes, not tens of minutes).
• N projects cost ~3.2 GB once + small deltas on disk, not N×3.2 GB.
• Reproducible + revocable: bump/pin the base once, rebuild → old agents/scanners stop everywhere.

Merge note

Touches the same Dockerfile region as #15 (AIDC_AGENTS) — they compose cleanly (this PR already passes AIDC_AGENTS through to the base build), but expect a trivial conflict on the agent block if both merge before rebasing.

Closes #7