fix: use ~/.config as the canonical macOS config path

[jy-tan]

Summary

Make ~/.config/fence/fence.json the canonical config location on macOS so new configs go where users expect, while preserving reads from legacy macOS paths until a real canonical config file exists. Also tighten the docs so Linux path guidance matches XDG_CONFIG_HOME and library examples behave like the CLI.

Resolves #75.

Changes

• Split config path handling into a canonical write path and a default load path with legacy fallback
• Update the CLI to load via ResolveDefaultConfigPath() so existing legacy macOS configs are still honored during migration
• Add resolver tests covering macOS canonical-path preference, legacy fallback, and Linux UserConfigDir behavior
• Export and document ResolveDefaultConfigPath() in the public package API
• Update library examples to use LoadConfigResolved() so relative extends paths resolve the same way as the CLI
• Refresh help text and docs to describe Linux in terms of $XDG_CONFIG_HOME and macOS in terms of ~/.config/fence/fence.json

[cubic-dev-ai]

2 issues found across 8 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="cmd/fence/main.go">

<violation number="1" location="cmd/fence/main.go:64">
P2: The new help text hardcodes `~/.config` on Linux, but the actual default follows `os.UserConfigDir()`/`XDG_CONFIG_HOME`, so it can document the wrong path.</violation>
</file>

<file name="internal/config/config.go">

<violation number="1" location="internal/config/config.go:175">
P2: Checking the canonical directory before legacy files can hide an existing legacy config. On macOS, an empty `~/.config/fence/` makes `DefaultConfigPath()` ignore `~/Library/Application Support/...` and `~/.fence.json` even when those files exist.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

3 issues found across 5 files (changes from recent commits).

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="docs/library.md">

<violation number="1" location="docs/library.md:86">
P2: Use `LoadConfigResolved` here so relative `extends` entries work the same way as the CLI.</violation>

<violation number="2" location="docs/library.md:101">
P3: This sentence overstates the legacy fallback. `ResolveDefaultConfigPath()` still returns the canonical path once `~/.config/fence/` exists, even if a legacy config file exists.</violation>
</file>

<file name="cmd/fence/main.go">

<violation number="1" location="cmd/fence/main.go:199">
P2: This can skip an existing legacy macOS config whenever `~/.config/fence/` exists but `~/.config/fence/fence.json` does not.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}