Add macOS platform support

[jwhonce]

Add Darwin build, loader, plugin, filesystem-watch, and mailbox polling support while retaining Linux inotify behavior.

Use kqueue for native macOS filesystem notifications, adapt dynamic-library conventions and Homebrew dependency paths, and cover macOS watcher/plugin behavior in tests. Restore the Linux mailbox path unchanged to avoid platform regressions.

Mount the local ONNX embedding model into the reusable Linux test container so the embedding test runs there, and propagate per-test failures through the test and test-container targets.

[Copilot]

Copilot review overview

🟡 Changes recommended

Unresolved critical build/runtime issues and moderate filesystem-watcher and test-container issues remain.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 3 · 2

Open (5)

• Homebrew linker paths are appended after library arguments · New
• Stale watches block re-registration after deletion or rename · New
• Fallback explicit_bzero conflicts with the system declaration · New
• Existing test containers lack the required model mount · New
• Non-recursive watches incorrectly become recursive · New

What changed in this PR

Adds macOS/Darwin support for builds, filesystem watching, plugins, polling, and related tests while preserving Linux behavior.

Changes:

• Adds Darwin build, linking, plugin, subprocess, journal, and polling support.
• Implements kqueue filesystem watching and macOS test coverage.
• Updates ONNX model discovery and reusable test-container handling.

File	Summary
tests/​test_tool_plugin_dlopen.c	Uses platform-specific plugin extensions.
tests/​test_onnx_embed.c	Supports current and legacy model paths.
tests/​test_fswatch.c	Extends filesystem watcher coverage for macOS.
src/​tools.c	Preserves plan-link compatibility and replay behavior.
src/​tool_plugin.c	Adds .dylib plugin recognition.
src/​telegram.c	Adds non-Linux polling behavior.
src/​subprocess.c	Adds macOS-compatible pipe and descriptor handling.
src/​matrix.c	Adds Darwin polling and credential clearing; critical: explicit_bzero may conflict with the SDK declaration.
src/​main.c	Integrates native watcher readiness and draining.
src/​mailbox.c	Adds macOS mailbox polling.
src/​journal.c	Uses fsync on macOS with error handling.
src/​fswatch_noop.c	Restricts the no-op backend to unsupported platforms.
src/​fswatch_kqueue.c	Adds kqueue watching; critical: stale deleted/renamed entries prevent re-registration. Moderate: non-recursive watches become recursive, hidden directories are not skipped, and directory events rescan entire subtrees.
Makefile	Adds Darwin builds and container testing; critical: Homebrew linker paths appear too late. Moderate: reusable containers may lack the model mount, and plugins lack a resolvable Darwin runpath.
.gitignore	Ignores macOS build artifacts.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Unresolved critical linking and filesystem-watcher safety issues remain, along with additional watcher and test correctness issues.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 2 · 1

Open (3)

• Installed macOS binary cannot resolve its libnash runtime dependency · New
• Avoid use-after-free when processing deleted watch entries · New
• Seed existing files in non-recursive directory watches · New

Resolved since last review (5)

• Fallback explicit_bzero conflicts with the system declaration
• Stale watches block re-registration after deletion or rename
• Homebrew linker paths are appended after library arguments
• Non-recursive watches incorrectly become recursive
• Existing test containers lack the required model mount

Previously missed (2)

In code that hasn't changed since last review

Recursive kqueue scan can exhaust file descriptors

src/​fswatch_kqueue.c:117

This recursive scan opens one kqueue vnode fd for every file and directory, with no depth or watch-count bound. The TUI calls this on the whole workspace, so a sufficiently large/deep tree can exhaust macOS file descriptors (after which watch_tree() silently skips paths) or recurse excessively; add bounds/resource handling comparable to the Linux backend or use a strategy that does not require one fd per file.

Canonicalize watch paths before generating callbacks

src/​fswatch_kqueue.c:177

The public callback contract requires absolute paths, and the Linux backend canonicalizes the input with realpath, but this backend stores path verbatim. Calling fswatch_add with a relative path therefore produces relative callback paths and can break consumers that compare callbacks with canonical workspace paths. Resolve the input before passing it to watch_tree/add_watch.