mcp: require prepared send intents

[darioAnongba]

Addresses P0-5 from #997.

Before:

• one MCP send call accepted payment parameters, prepared the payment, and immediately dispatched it
• an agent had no tool boundary at which to inspect the resolved amount, rail, fees, destination, or warnings before funds could move

After:

• send.prepare accepts payment parameters, validates them, and returns the daemon's exact preview plus a short-lived single-use send_intent_id
• send accepts only send_intent_id, consumes the already prepared intent, and may move funds
• tool schemas and tests pin that prepare cannot dispatch and dispatch cannot reconstruct or change reviewed parameters
• the runtime registry exposes send.prepare as MCP-only and advertises the distinct MCP parameters for send
• disabled wallet clients return the normal wallet-RPC-disabled error instead of risking a nil dereference
• operator documentation explains the two calls

Concrete consequence: MCP agents must observe the resolved transaction before authorizing dispatch, and the final call cannot silently substitute a different destination, amount, rail, or fee limit.

This reuses the daemon's existing send-intent mechanism; it adds no new persistence or approval subsystem.

Validation:

• make unit pkg=./cmd/wavecli/...
• go test ./cmd/wavecli/... -race
• make lint-changed-local
• make commitmsg-lint range="origin/main..HEAD"

This PR is independent from the other #997 slices and targets main.

[gemini-code-assist]

Code Review

This pull request refactors the MCP wallet payment process into a two-phase flow by splitting the single send tool into send.prepare (which validates and returns a send_intent_id) and send (which dispatches the payment using the intent ID). This change is documented across several markdown files and is supported by new unit tests. The review feedback suggests adding nil checks for the client interface in both prepareMCPWalletSend and dispatchMCPWalletSend to prevent potential nil pointer dereference panics, especially during testing.

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[chatgpt-codex-connector]

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