* fix(ai-vault): resume a bridged Codex session under the selected account's home
The account session bridge hardlinks every rollout into each per-account
CODEX_HOME, and vault dedup keeps the lexicographically-smallest alias, so
Resume could pin an inline CODEX_HOME naming a peer account — running the
session under that account's auth.json and quota. At resume time the owning
host now substitutes the selected account's home when it holds the same
rollout at the same sessions-relative path, declining on any uncertainty so
resume degrades to today's behavior instead of failing.
* fix(ai-vault): repin dropped sessions without a cwd instead of resuming under the wrong account
The drag payload only carried sessionCwd when session.cwd was truthy, so a
null-cwd codex session dropped onto a pane silently fell back to the prebuilt
command - which pins the wrong account's CODEX_HOME, the exact defect this PR
eliminates on the other resume surfaces.
- Serializer always sends sessionCwd (null when the session has no cwd), so
absence now only means an older-serializer payload.
- The repin rebuild accepts a null cwd (the builders already omit the cd
prefix), matching the sidebar Resume/Copy paths which repin regardless of cwd.
- An unrepinnable payload (absent sessionCwd) now fails loudly with guidance
instead of silently resuming under the wrong account's home.
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