feat: support template interpolation in multi-agent usage hints

[jif-oai]

Why

multi_agent_v2 usage hints sometimes need to reference resolved config values such as the effective thread limit. Those values only exist after config layering, defaulting, and feature materialization, so the raw TOML alone was not enough to render them.

What changed

• allow features.multi_agent_v2.{usage_hint_text,root_agent_usage_hint_text,subagent_usage_hint_text} to use {{ ... }} placeholders backed by the materialized effective config
• fail config loading with a targeted error when a referenced placeholder does not exist or does not resolve to a scalar value
• move resolved-config materialization into a shared helper so config interpolation and config-lock export/replay both serialize the same resolved feature, memory, and agent settings

Example

[features.multi_agent_v2]
enabled = true
usage_hint_text = "lorem {{ features.multi_agent_v2.max_concurrent_threads_per_session }} ipsum"

gets rendered as

        "description": String("... \lorem 4 ipsum"),

[jif-oai]

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[chatgpt-codex-connector]

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