fix(delivery): preserve exact identity pairs in check-inbox

[suiren]

Summary

Derive check-inbox.sh's delivery targets from identities.sh's exact
(team, agent) rows instead of whoami.sh's flattened human-readable
output, so the Stop hook never polls — or marks read — a (team, agent)
pair that was never registered.

Problem

identities.sh returns the registered pairs as team<TAB>agent rows:

alpha	alice
beta	bob

whoami.sh flattens those rows into independent lists for its
human-readable output:

multiple=true agents=alice,bob teams=alpha,beta

check-inbox.sh parsed that output, taking the first agent and
every team. With multiple identities registered for the same
(project, type) it therefore polled the cross product — here
(alpha, alice) and (beta, alice) — although (beta, alice) was
never registered. Any message row matching such a phantom pair is
displayed to the wrong session and marked read, so its actual
addressee never sees it.

Change

Consume identities.sh's TSV directly:

• Keep the existing first-agent policy, but accumulate only that
  agent's actual team rows — no cross-product pair is ever queried or
  marked read.
• Keep the project-root resolution the whoami path performed
  internally: agmsg_resolve_project runs before the lookup, since
  identities.sh itself is an exact registry lookup by design (its
  other callers depend on that). Without it, a session started from a
  nested subdirectory or a sibling worktree would silently stop
  receiving messages.
• Empty or malformed identity output delivers nothing, matching the
  previous not_joined/suggest behavior.

Unchanged: the cooldown marker stays keyed by the selected agent, the
per-pair actas exclusivity check still runs inside the team loop, and
the #653 loop-failure capture (CLAIM_RC) is untouched.

The empty-array expansion in the dedup loop is guarded with the
${arr[@]+...} idiom already used in resolve-project.sh, since the
script runs under set -u and macOS CI runs bash 3.2.

Test plan

Two regression tests in tests/test_inbox.bats:

1. With (alpha, alice) and (beta, bob) registered for the same
   project and type, only (alpha, alice) is displayed and marked
   read; both cross pairs and the second agent's row stay unread.
   Reverting the fix turns it red: the old parser polls
   (beta, alice) and falsely marks it read.
2. With a registration at the project root, an invocation from a
   nested subdirectory still resolves to the root and delivers —
   guarding the resolution step, which no existing test exercised at
   the check-inbox call site.

Honest status: the suite is expected to run in this repo's PR CI; it
was not executed locally on our side (Windows dev machine, as with
#613).

Note for integration/remote

The same flatten-and-cross-product block exists unchanged on
integration/remote (the storage rework there did not touch the
identity derivation), so this fix will want a small port when that
branch lands on main. Both regression tests should carry over as-is.

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