fix: move pre-existing .devcontainer aside instead of refusing to init

[ai-anant]

Summary

Closes #6. When a repo already ships its own .devcontainer/ (or an aidc-managed .cursor/rules/00-core-logics.mdc), aidc init / aidc claude aborted with:

[aidc] error: refusing to overwrite existing file: <PATH>/.devcontainer/devcontainer.json

This made aidc unusable on any repo that already had a devcontainer — exactly the scenario in the issue ("AIDC wont initialize if .devcontainer already exists").

Change

lib/aidc.sh — aidc::check_init_conflicts() no longer dies when a pre-existing managed path is found. Instead it:

• Detects any aidc-managed path under the top-level dirs aidc scaffolds wholesale (.devcontainer/, .cursor/rules/).
• Moves the whole top-level dir aside to <dir>.aidc-backup (numeric suffix .1, .2, … on collision) — user content is renamed, never deleted.
• Prints a restore hint: mv .devcontainer.aidc-backup .devcontainer.
• Then refresh_scaffold proceeds normally.

aidc::ensure_local_git_excludes() now also excludes .devcontainer.aidc-backup*/ so the backup dir doesn't pollute git status.

Behavior matrix:

Pre-existing state	Before	After
.devcontainer/devcontainer.json (typical VS Code devcontainer)	error: refusing to overwrite	moved to .devcontainer.aidc-backup, scaffold proceeds
.cursor/rules/00-core-logics.mdc	error: refusing to overwrite	.cursor moved to .cursor.aidc-backup, scaffold proceeds
.cursor/rules/my-user-rule.md (no aidc-managed path)	scaffold proceeds	unchanged — user rule kept alongside aidc's 00-core-logics.mdc
Already initialized (.ai-container/project.env present)	no-op	no-op (idempotent, re-init clean)

Verification

• bash -n under bash 3.2.57 (the project floor) — clean
• shellcheck --severity=warning on bin/aidc, lib/aidc.sh, all *.sh (mirrors shellcheck.yml) — clean
• .github/scripts/bash-compat-check.sh under bash 3.2 — parse + smoke OK
• tests/resolve-oauth-token.test.sh — 7/7 pass
• .github/scripts/test-bootstrap-state.sh — 9/9 pass
• python3 -m py_compile bin/aidc-clipboard-server — OK
• End-to-end simulation (real aidc init): repo with pre-existing .devcontainer/ (devcontainer.json + scripts) + .cursor/rules/my-user-rule.md → init succeeds, originals preserved in .devcontainer.aidc-backup, user cursor rule untouched, scaffold complete. Second scenario with a .devcontainer.aidc-backup already present → backup correctly lands in .devcontainer.aidc-backup.1. Re-running aidc init on an initialized repo stays a clean no-op.

Fixes #6