Summary

Ports two per-channel overrides that the in-process Telegram adapter had but the sidecar architecture dropped (#5241), so sidecar operators can replicate them again.

Closes #5841.

1. Command policy — command_policy on [[sidecar_channels]]

Re-exposes the in-process disable_commands / allowed_commands / blocked_commands gating (#2063) on sidecar channels:

[[sidecar_channels]]
name = "afina-sales-bot"
command = "python3"
args = ["-m", "librefang.sidecar.adapters.telegram"]
channel_type = "telegram"
command_policy = "disable"      # allow (default) | disable | allowlist | blocklist
# allowed_commands = ["start"]  # used when command_policy = "allowlist"
# blocked_commands = ["new", "reboot", "agent"]  # used when command_policy = "blocklist"

A rejected command is forwarded to the agent as plain text, never answered with an "unknown command" error — preserving the #2063 behaviour public-facing / customer-service bots rely on.
A non-technical end user who types /new or /reboot no longer triggers a session reset.

2. Inbound coalescing — message_coalesce_window_ms

Re-exposes the inbound debounce window (#4441):

[[sidecar_channels]]
name = "afina-sales-bot"
message_coalesce_window_ms = 3000   # buffer inbound messages; dispatch once window expires

Messages from one sender arriving within the window are buffered and dispatched as a single batched context, so a user forwarding several messages in rapid succession produces one agent invocation instead of racing concurrent ones on the same session (the "Sorry, something went wrong" regression in the issue).

Implementation

Reuses the gating the bridge already has — the inbound is_command_allowed check (bridge.rs) and the MessageDebouncer.
The only gap was a config path to feed them for sidecars, since KernelBridgeAdapter::channel_overrides() returns None for sidecar channels.

• SidecarChannelConfig gains command_policy: SidecarCommandPolicy, allowed_commands, blocked_commands, message_coalesce_window_ms — all #[serde(default)], backward-compatible.
• SidecarAdapter builds a ChannelOverrides from its block (overrides_from_sidecar_config) and exposes it via a new ChannelAdapter::channel_overrides() default trait method (default None).
• The bridge prefers that per-instance override over the kernel-level channel_overrides(channel_type, …) lookup at all three resolution sites (debouncer setup in start_adapter, dispatch_message, and the coalesced-media flush path). The kernel lookup is keyed only by channel_type and cannot distinguish two sidecars sharing a channel_type (e.g. two Telegram bots); a per-adapter override can.
• command_policy maps onto the existing is_command_allowed boolean/list fields; message_coalesce_window_ms maps onto the debouncer's message_debounce_ms. Gating happens uniformly kernel-side, so no Python sidecar change is needed — the adapter still emits the command, the bridge forwards it as text when policy rejects it.

Tests

• librefang-types (config/types.rs): serde defaults are backward-compatible (allow policy, coalescing off); disable / allowlist / blocklist blocks parse with their command lists.
• librefang-channels (sidecar.rs): overrides_from_sidecar_config maps each policy variant and the coalesce window correctly, returns None for a plain config (so the kernel fallback still applies), and SidecarAdapter::channel_overrides() surfaces the built overrides.

Verification

Run in the sanctioned Docker dev image (Dockerfile.rust-dev, named volumes — host target/ untouched):

• cargo test -p librefang-types -p librefang-channels --lib — 1333 pass (6 new in channels, 3 new in types)
• cargo clippy -p librefang-types -p librefang-channels --all-targets -- -D warnings — clean
• cargo fmt -p librefang-types -p librefang-channels -- --check — clean

Notes / deferred

• The kernel-config golden fixture (kernel_config_schema.golden.json) was updated by hand to mirror the deterministic schemars output (new command_policy / allowed_commands / blocked_commands / message_coalesce_window_ms properties + the SidecarCommandPolicy definition). The automated regen test lives in librefang-api, whose build currently hits a pre-existing aarch64-Linux libc::SYS_getpgrp break in librefang-runtime (unrelated to this change; SYS_getpgrp is x86-only). The golden assertion kernel_config_schema_matches_golden_fixture is itself #[ignore]d on main for separate pre-existing drift, so CI does not gate on the fixture yet — but please re-run cargo test -p librefang-api --test config_schema_golden -- --ignored regenerate_golden --nocapture on an x86_64 host to confirm a byte-match if convenient.
• No CHANGELOG entry added (no [Unreleased] section exists on main and the attribution guard expects (@user)); add one on merge.
• No dashboard form for these fields — they are TOML-only, matching the issue's proposed config shape. Adding a dashboard surface would be a separate change in the dashboard domain.