feat(cli): warn when --name disagrees with the resolved identity (#148)

* feat(cli): warn when --name disagrees with the resolved identity

An explicit --name overrides whatever the shell is bound to. That is
deliberate, and it is also how an identity drifts unnoticed: a session keeps
sending under the name its bootstrap announced while its row has since been
rebound or recovered under another one. bootstrap.rs tells every agent to pin
that name into every command, so the pin outlives the thing it names, and
nothing said so. The first sign was a peer replying to a name that no longer
routed anywhere.

CommandContext carries an identity_warning that the router prints to stderr:

  [hcom] warning: sending as 'riko', but this shell resolves to 'voni'.
  If 'riko' is stale, run 'hcom start --as riko' to reclaim it.

It warns rather than refuses. A human at a terminal, and a relay acting for a
remote instance, both legitimately pass a name that is not their own, so
failing the command would break working setups to catch a rarer mistake.

Two cases are exempt, both because the shell's own binding is not a second
opinion worth weighing there. A subagent shares the parent's session and
shell, so every subagent command resolves to the parent row. A verified
Claude actor is already the exact acting agent for the call, and one acting
through the actor env legitimately differs from the row its process binding
names; comparing them would manufacture a warning on every call.

Closes #140.

* Keep --name drift check read-only and scoped

- Read the process binding directly instead of resolve_identity, whose
  Codex recovery path can rebind or retire rows.
- Only check when --name is passed; skip printing with --from.
- Exempt only subagents of the bound row.
- Word the warning as a name/binding mismatch; --name is the agent's
  own truth, so keep the start --as reclaim hint.

* Only treat send --from before -- as an external sender

has_from_flag scanned message text after -- too, which also let such a
send bypass the identity gate. The drift warning is now only suppressed
for send, since listen/events --from is a filter.

* Recognize --from=NAME as an external sender

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Co-authored-by: aannoo <aannoo@users.noreply.github.com>