feat(commands): render agents under their display name in room commands

[raullopez-sandbox]

Stacked on #338. Review that one first; this diff is only the last commit.

What

The ! command surface was the last place still naming every agent by its
routing identifier. A room that shows "Switch Dev" in every bridged message
dropped back to "switchdev" the moment anyone ran !list-agents.

Which name a command prints is decided by one question — does the reader
have to type it back?

Surface	Prints	Why
!list-agents, !list-all-agents, !invite-agent, !alias, status replies	Switch Dev (`switchdev`)	the identifier is the thing you type next
!roles holders, !list-documents creators	Switch Dev	nobody addresses a document's author
room greeting	@switchdev	it is a live mention token and has to stay one

Listings sort on the label, so the order matches what is on screen rather than
a hidden identifier.

The security half

A display name is text a user typed, and these replies are handed to the
bridge, which escapes nothing on the way out but does rewrite a plain
@handle into a real mention. A display name of @everyone would have been a
mass ping — latent before this feature existed, armed by it.

Every command therefore defuses the label first. The zero-width-space
treatment the platform adapters already applied moves down into
agent_display_name so the command layer shares that one implementation
instead of growing a second copy:

stored : '@everyone'
posted : '- **@​everyone (`switchdev`)**'

Invisible to a reader, inert to the mention parser.

Verification

2237 pass (+47), ruff and mypy clean. Beyond the suite I mutation-tested the
six load-bearing claims — every one of them fails the suite when broken:

• defusal reduced to identity → 41 failures (the bridge suite goes with it,
  which is what proves the extraction is genuinely shared rather than a copy)
• agent_label / agent_label_with_identifier skipping defusal → 6 / 14
• an agent with no display name rendering switchdev (`switchdev`) → 12
• sort key reverting to the identifier → 2
• a routing site switched to the display name → 1

That last one is the one worth keeping: it is the check that stops a later
edit from "consistently" applying the display name to the greeting and
silently breaking the mention it hands out.

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