feat(mold): auto-discover well-known ai-agent directories during cast

[kriscoleman]

When a mold's only content lives under a dot-prefixed top-level directory (e.g. .claude/, .cursor/, .codex/), ailloy cast silently installs nothing because discoverTopLevelDirs in pkg/mold/output.go:168 excludes any entry whose name starts with .. Authors must add an explicit output: mapping in flux.yaml like:

output:
  .claude: .claude

This is a sharp edge — the canonical Ailloy use case is shipping commands and rules to AI coding agents, all of which conventionally live in dot-prefixed directories. A new mold author who scaffolds .claude/commands/foo.md, runs cast, and sees an empty target directory has no signal that anything went wrong.

This was found during end-to-end testing of the foundry index system in PR #81 against a private kriscoleman/foundry-test repo: a minimal mold with only .claude/commands/hello.md produced an "✅ Project casting complete!" message but installed no files.

Proposed Change

Add an allowlist of well-known AI-agent directories to pkg/mold/output.go that are auto-discovered despite their dot-prefix. Initial entries:

• .claude (Claude Code)
• .cursor (Cursor)
• .codex (Codex)
• .opencode (OpenCode)
• .windsurf (Windsurf)
• .continue (Continue)
• .aider (Aider)
• .github (GitHub Copilot lives at .github/copilot-instructions.md, plus shared workflow blanks)
• .gemini (Gemini CLI)

Behavior:

• A mold whose only top-level content is one of these dirs will install correctly with no flux.yaml output mapping.
• An explicit output: mapping in flux.yaml continues to override auto-discovery (no behavior change for existing molds).
• Other dot-prefixed dirs (e.g. .git, .idea, .vscode-test) remain excluded.
• The same allowlist should be applied to discoverRootFiles for analogous root-level dotfiles where it makes sense (e.g. .cursorrules, .aider.conf.yml) — confirm during implementation whether that's symmetric.

Acceptance Criteria

• pkg/mold/output.go exports a named allowlist (e.g. wellKnownAgentDirs) alongside reservedDirs with the directories listed above.
• discoverTopLevelDirs includes a dot-prefixed dir if and only if its name is in the allowlist.
• A mold containing only .claude/commands/hello.md (no flux.yaml) installs .claude/commands/hello.md to the project root via ailloy cast.
• Same passes for .cursor/, .codex/, .opencode/, .windsurf/, .continue/, .aider/, .github/, .gemini/.
• A mold containing .git/ or .idea/ is still excluded from auto-discovery.
• Existing molds that use explicit output: mappings continue to behave unchanged.
• Unit tests in pkg/mold/output_test.go (or equivalent) cover both allowlisted and non-allowlisted dot-prefixed dirs.
• If discoverRootFiles gets symmetric treatment, well-known root dotfiles (.cursorrules, etc.) are covered by tests too.
• assay lint output mentions when a mold relies on the allowlist, so authors aren't surprised (optional — confirm during implementation).

Notes

• Source pointer: the exclusion lives at pkg/mold/output.go:168 (strings.HasPrefix(name, ".")) and at :190 for root files.
• Keep the allowlist as a single source of truth — both auto-discovery paths (identity mode and the string output: parent form) use discoverTopLevelDirs, so a single change covers both.
• Consider whether the allowlist should be configurable via mold.yaml (autoDiscover: [".myagent"]) for forward-compat with new tools — likely overkill for v1, but worth a sentence in the PR description.
• Discovered while testing PR feat: foundry index system for SCM-agnostic mold discovery #81: feat: foundry index system for SCM-agnostic mold discovery #81