test: never write the real user state dirs from tests

[Emasoft]

Several tests were quietly writing to the real user state dirs: session-meta sidecars landed in ~/.local/share/late/sessions, and plugin management tests rewrote the global plugins.json under ~/Library/Application Support/late. This branch redirects all of them to temp dirs, so the suite provably writes nothing there.

• Session-meta sidecars — 10 test functions (agent/orchestrator/executor) that spawn subagents or commit turns now point the session layer's global SessionDir at t.TempDir(). Production meta placement is unchanged (the --continue lister depends on it).
• Global plugin state — 10 test functions (link/install/enable/disable/remove flows) now run under the package's sandboxUserConfig convention, so plugins.json is always a throwaway.
• Test-only: no new tests, no production changes.

Note: the sibling feature PRs split from the same upstream base and don't carry these redirects — this branch holds the full isolation set on its own.

Testing

Full suite run against the real home with before/after sentinels on both state dirs:

$ go test ./... -race -count=1
ok  	late/cmd/late	2.430s
?   	late/cmd/mcp-run	[no test files]
ok  	late/internal/agent	4.504s
?   	late/internal/assets	[no test files]
ok  	late/internal/client	1.446s
ok  	late/internal/common	2.954s
ok  	late/internal/config	2.886s
ok  	late/internal/executor	9.911s
ok  	late/internal/git	2.873s
ok  	late/internal/mcp	1.778s
ok  	late/internal/orchestrator	4.893s
?   	late/internal/pathutil	[no test files]
ok  	late/internal/plugin	14.225s
ok  	late/internal/session	3.651s
ok  	late/internal/skill	4.424s
ok  	late/internal/tool	1.806s
ok  	late/internal/tool/ast	4.551s
ok  	late/internal/tui	10.671s

exit 0 — all 18 packages green (15 ok, 3 without test files).

Proof of no user-dir writes:

$ find ~/.local/share/late ~/Library/Application\ Support/late -type f -newer <pre-test sentinel>
(no output)

newer-count: 0

• Full path+mtime+size listing of both dirs: 499 files before, 499 after, diff empty (tree identical)

• config.json sha256 before and after: 70fa8c8f7fc750b18b9de0513adf599ca2f766d9ac5ebeca51edcc25267eb2a9 (unchanged)

• gofmt -l on the touched files: no output (clean)

• go vet ./internal/agent ./internal/executor ./internal/orchestrator ./internal/plugin: clean

[mlhher]

Thank you for splitting the PRs that will allow going far quicker at them.