docs(rfd): Add RFD 107 discoverable tools

[JeanMertz]

RFD 107 proposes a third tool enablement state. A discoverable tool sits in the catalogue and is permitted, but its definition is withheld from the model until a search_tools builtin finds it, at which point a ConfigDelta promotes it to offered for the rest of the conversation. This keeps a large MCP catalogue out of the context window without hiding capability from the user, and it works on every provider; only cache preservation across a discovery is Anthropic-specific.

The design turns on two boundaries. The single Vec<ToolDefinition> that today serves the provider array, the describe_tools docs map, executor eligibility, and MCP startup splits into a catalogue and an offered subset, because a discoverable tool has to be searchable and callable while staying out of the provider's tools array. Promotion crosses the boundary RFD 078 closed against builtin config mutation, so it travels a host-owned channel carrying only tool names rather than outcome.config, and is folded against the state at its own position so it cannot silently revert a disable the user approved earlier in the same cycle.

RFD 105 gains a paragraph recording why deferring tool search is not neutral: its initial directive offers every enabled tool and withdraws every disabled one, leaving discovery nowhere to operate until the third state exists. The relationship is recorded as Extends on both RFD 081, whose enablement model 107 widens, and RFD 078.