feat(send): --all broadcast and comma-separated recipients

[kzv8wznnj4-cloud]

What

Extends <to> in send.sh <team> <from> <to> <message> to support multiple
recipients and broadcast:

• --all (alias @all) — deliver to every team member except the sender
• alice,bob — comma-separated multiple recipients (spaces after commas are
  tolerated, empty entries are skipped)
• Single-recipient calls are byte-for-byte unchanged (no regression)

Why

Broadcasting to a whole team (leader → all members) is a common need, but
previously required calling send.sh once per recipient. This addresses the
long-standing requests in #25 / #26.

How

• --all/@all: enumerates agents from the team config
  (teams/<team>/config.json) via sqlite3's json_each, excluding the
  sender, to build the recipient set.
  • The team name is validated with agmsg_validate_team_name
    (lib/validate.sh), rejecting path fragments that would escape
    teams/ (Team names can escape teams/ via path traversal #140).
  • tr -d '\r': works around Windows sqlite3.exe emitting CRLF-terminated
    rows (same class of issue as team.sh: CRLF in sqlite3 row output breaks member-count arithmetic on Windows (Git Bash) #130).
• Comma-separated recipients: split with IFS=',' read -r -a, then each
  recipient is trimmed of surrounding whitespace and empty entries are
  dropped, so alice, bob, alice,,bob, and alice, all resolve cleanly.
  If every entry is empty the call fails with Missing to agent.
• Delivery is a single multi-row INSERT — atomic, so a partial fan-out
  (some recipients get the message, others don't) can't happen if one row
  fails mid-way.
• Every interpolated value (team/from/to/body) is escaped via
  _agmsg_sqlesc (no new SQL-injection surface introduced).

Breaking changes

None. Single-recipient invocations produce byte-identical stdout and DB
writes to before this change; no new required flags, no changed defaults.

Tests

Added 9 cases to tests/test_messaging.bats covering broadcast (--all and
@all), team-not-found and sole-member failure modes, comma-separated
recipients, space tolerance after commas, skipping empty middle/trailing
entries, and single-recipient backward compatibility.

Behaviour was also verified functionally end-to-end against an isolated store
(AGMSG_STORAGE_PATH): alice, bob / alice,,bob / alice, all deliver to
the intended recipients, , , fails with Missing to agent, and @all /
--all broadcast to every member except the sender (a sole-member team fails
with No recipients).

Compatibility

• Existing single-recipient call output and DB writes are unchanged.
• Verified on Windows (Git Bash), including the CRLF workaround (team.sh: CRLF in sqlite3 row output breaks member-count arithmetic on Windows (Git Bash) #130-class
  issue).