fix(kernel): lenient binding parsing — one typo no longer drops the entire bindings table

[benhoverter]

Summary

Fixes #1145.

A single misspelled field anywhere in [[bindings]] currently causes the entire bindings table to fail parsing at config load. Because KernelConfig deliberately does not use deny_unknown_fields at the top level (forward-compat with new sections), the failure surfaces as a silent default: every agent ends up unbound, the daemon boots clean, and routing simply stops working — with no log line pointing at the typo.

This PR makes binding parsing partial-success: each entry is parsed independently, malformed entries are dropped with a clear ERROR naming the agent and the bad field, and survivors load normally. Per-entry deny_unknown_fields is preserved, so silent typos still fail loudly per entry — just no longer catastrophically across the whole table.

Changes

• New lenient_extract_bindings(&toml::Value) in openfang-kernel/src/config.rs runs after include-merge / [api] migration, before the main try_into::<KernelConfig>() call. Each [[bindings]] entry is try_into::<AgentBinding>()'d on its own; failures log an ERROR with index, best-effort-extracted agent name, and the underlying serde error message (which already lists valid field names). A single aggregate WARN reports dropped vs. surviving counts. Survivors are reinjected into the config value before the normal deserialize proceeds.
• No changes to AgentBinding or BindingMatchRule schemas — deny_unknown_fields is preserved on both. Behavior change is scoped strictly to the failure mode of the bindings array.
• TODO marker added on the remaining warn!-on-load fallback in load_config for the non-binding silent-default path (follow-up work).

Out of scope: the broader silent-default path for non-binding config sections; surfacing the dropped-binding count in any non-log channel.

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes (+7 new tests, all green)
• Live integration tested

Live smoke test on a daemon built from this branch stacked on a hardened base: typo'd a binding field (cccchannel) in config.toml and bounced the daemon. Log output:

ERROR Skipping malformed binding #3 (agent='assistant-teo'):
      unknown field `cccchannel`, expected one of
      `channel`, `account_id`, `peer_id`, `guild_id`, `channel_id`, `roles`
WARN  Dropped 1 malformed binding(s); 11 binding(s) will load.
INFO  Loaded agent bindings into router count=11

The error message names the agent, the typo'd field, and the valid alternatives. Surviving bindings loaded and routed correctly.

7 new unit tests cover:

• test_lenient_bindings_drops_typo_keeps_rest — the issue reproducer.
• test_lenient_bindings_all_valid_unchanged — happy path: no behavioral change for clean configs.
• test_lenient_bindings_all_malformed_yields_empty_but_keeps_rest_of_config — every binding malformed → empty bindings, but the rest of the config still loads.
• test_lenient_bindings_no_bindings_section_is_noop — configs without [[bindings]] are untouched.
• test_lenient_bindings_missing_agent_field_dropped — entry without an agent field is dropped (cannot be routed).
• test_lenient_bindings_preserves_survivor_order — locks in that first-match-wins routing semantics cannot silently regress when a middle entry is dropped.
• test_lenient_bindings_top_level_field_typo_dropped — locks in that deny_unknown_fields catches operator typos at the binding top level (e.g. agnt = ...), not just inside match_rule.

CI note: the same pre-existing cargo fmt --all -- --check failure on main (tracked by #1121, inherited from da6b567a and earlier) applies here. None of the failing fmt lines are in crates/openfang-kernel/src/config.rs — the only file this PR modifies. Happy to address as a separate fmt-only PR if maintainers prefer.

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries
• No personal agent names in diff, description, tests, or fixtures

Note on relationship to #1144

This PR pairs naturally with #1144 (channel_id binding hardening). Together they make binding misconfig caught early (#1144) and survivable (this PR) instead of silent-and-catastrophic. The two PRs touch disjoint code paths and can land in either order.