fix(agent): refuse an implicit HOME cwd for a directory spawn

`codeman agent spawn` always sends its directory as an explicit
workingDir, so the server cannot apply its no-workingDir rule and
installs workspace hooks (and an injected skill) into ~/.claude when the
caller's cwd is HOME. A bare spawn whose cwd resolves to the home
directory now refuses with exit 4 before any request and names
`--working-dir ~`; an explicit `--working-dir ~` is still sent. Paths
are compared by realpath, so a symlinked HOME matches, and an
unresolvable HOME or cwd falls back to the resolved path without
throwing.

The directory-spawn --json shape now reads the same in the README, the
API reference, the wiki page and the changeset: `ready` when a
readiness wait ran, `readinessError` when that call failed, and
`startupError` when the start failed. A test pins all four.

plugins/codeman/skills/codeman/SKILL.md

@@ -46,7 +46,8 @@ For an existing project, use one command:
 no `--working-dir`, `spawn` uses cwd. Give editing workers separate worktrees. A
 positional case keeps quick-start behavior and cannot be combined with
 `--working-dir`; an empty `--working-dir` (`"$WT"` unset) refuses with exit 4
-rather than falling back to cwd. Directory launches keep parent lineage and never
+rather than falling back to cwd, and a cwd that is HOME refuses too (hooks would
+land in `~/.claude`; pass `--working-dir ~` to mean it). Directory launches keep parent lineage and never
 mark the project as agent scratch. If startup fails or is ambiguous, the CLI
 reports the exact created ID: inspect it before retrying or cleaning up.