Closes #5294.

Problem

Sidecar adapter migration (#5241) dropped the per-channel default_agent field that in-process configs (TelegramConfig, DiscordConfig, …) used to seed AgentRouter.channel_defaults at boot. The router-population loop in channel_bridge.rs (around line 3295) pushes sidecar adapters into the adapters vec with None for the default-agent slot, so the per-channel router.set_channel_default_with_name call never fires for sidecar channels.

Result: inbound messages on a sidecar channel with no explicit binding fall through to the non-deterministic 'first available agent' branch in resolve_or_fallback. Spawning a new agent (without restricting its channels allowlist) silently redirects inbound traffic to it.

Reproduction

1. Configure Telegram via [[sidecar_channels]]. Existing agent (e.g. fandangorodelo) with channels = ["telegram"] in its manifest, receiving Telegram traffic correctly.
2. Create a second agent (any purpose, e.g. Profesor) without restricting its channels list (defaults to channels = []).
3. Send a Telegram message → ends up routed to Profesor instead of fandangorodelo, because the router has no channel_defaults["telegram"] and falls through to first-available.

Fix

Add default_agent: Option<String> to SidecarChannelConfig (mirrors the equivalent field on in-process channel configs) and propagate it through the adapter-push so the existing population loop seeds the router.

[[sidecar_channels]]
name = "telegram"
command = "python3"
args = ["-m", "librefang.sidecar.adapters.telegram"]
channel_type = "telegram"
default_agent = "fandangorodelo"   # NEW

No behavioural change when the field is absent — deployments that currently rely on the "first available" fallback continue working.

Test plan

• cargo check -p librefang-types -p librefang-channels — clean
• cargo clippy -p librefang-types -p librefang-channels --all-targets -- -D warnings — clean
• cargo test -p librefang-channels --lib sidecar_default_agent_roundtrip_5294 — passes (new test covering absent + explicit-value round-trip).
• Manual repro on live deploy (rodela-ai fork): without fix, traffic routed to wrong agent; with fix + default_agent = "fandangorodelo" in config, traffic routes correctly even when other agents exist.

Notes

This only addresses the missing-router-default half of #5294. The two related concerns (channels = [] accept-all semantics, and the 'first available' fallback being a silent footgun) are separate and need their own discussion — see the issue body. Happy to follow up on either if maintainers want to flip those defaults too.