feat: Linux support — graceful automation skip on non-macOS (fixes #31)

[YasienDwieb]

Fixes #31.

Problem

codealmanac setup --yes (and config set automation.*) crash on Linux:

codealmanac: launchctl bootstrap failed for com.codealmanac.sync: [Errno 2] No such file or directory: 'launchctl'

create_app unconditionally wires AutomationService with LaunchdSchedulerAdapter, which shells out to macOS launchctl. The crash also left a half-configured global state (config.toml written before the scheduler failed).

Approach — Option A: graceful skip, not a rewrite

Every lifecycle/read command is already platform-neutral; the only macOS blocker is scheduled automation, and SchedulerAdapter is already a port. So:

• UnsupportedSchedulerAdapter — a no-op implementation of the existing SchedulerAdapter port (no subprocess, no ~/Library/LaunchAgents writes; reports "not installed").
• core/platform.scheduler_supported() — single platform predicate. create_app selects launchd on macOS, the unsupported adapter elsewhere. Platform detection lives only at the composition root.
• Render reflects the outcome, not the platform. AutomationTaskApplyResult gains a scheduled field set from the adapter's real result (install().installed). Setup output now says "Scheduled automation unavailable" with a "run sync/garden manually" pointer instead of falsely claiming schedules were installed. Render stays free of sys.platform checks.

Because nothing fails on the unsupported path, the half-configured-state bug is gone as a side effect — setup completes and exits 0.

Real Linux scheduling (systemd user timers) is deliberately left as a follow-up (Option B); this PR unblocks all Linux usage today.

Verification

• uv run ruff check . clean; 570 tests pass (6 new in tests/test_linux_support.py, platform forced via sys.platform so they're host-independent).
• Smoke-tested on real Ubuntu: the former crash path (config set automation.sync.enabled true) exits 0, automation status works, no launchd artifacts created.
• macOS output is unchanged: launchd's install().installed is True, so every existing render branch is taken as before.

A design writeup is in docs/plans/2026-07-24-linux-support-option-a.md.