fix(sdd): ownership markers stop same-basename plans sharing a workspace

sdd-workspace slugged workspaces by basename alone, so docs/alpha/plan.md
and docs/beta/plan.md resolved to one directory and task-brief silently
overwrote the other plan's brief — the single gitignored source of task
requirements, unrecoverable once clobbered.

Each workspace now records its owning plan in a plan-path marker
(repo-relative in-repo, absolute outside). Lookup keeps basename slugs
and existing behavior for the common case: a markerless workspace is
adopted in place (no migration break for in-flight plans), a marker
naming this plan is a match, and a marker naming a different plan
disambiguates with the plan's parent-directory name, then a counter.
Plan paths are normalized (CDPATH-guarded physical cd) so relative,
absolute, and ../ spellings of one plan share one workspace.

task-brief and review-package delegate to sdd-workspace and need no
changes. SKILL.md's workspace bullet no longer promises the exact
<plan-basename> path, since disambiguated workspaces differ.

Reported by @CRGDan; reproduction and test groundwork by @crisnahine
in PR #2120.

Fixes #2045

skills/subagent-driven-development/SKILL.md

@@ -135,7 +135,7 @@ a ledger file, not only in todos.
 
 - Each plan owns a workspace: at skill start, run this skill's
   `scripts/sdd-workspace PLAN_FILE` — it prints the plan's git-ignored
-  directory (`<repo-root>/.superpowers/sdd/<plan-basename>/`), home to
+  directory (under `<repo-root>/.superpowers/sdd/`), home to
   every artifact for THIS plan: ledger, briefs, reports, review packages.
   Another plan's directory is never yours to read or write.
 - Check for this plan's ledger at `<workspace>/progress.md`. If its first