feat(mcpgen,mcpserver): templated @mcpTool selections

[zer0stars]

Summary

• @mcpTool(selection: "...") now accepts Go text/template syntax. The rendered selection is spliced into the query at call time with the call's argument map as template data.
• Non-templated selections keep taking the existing fast path; no breaking change for any existing tool.
• Adds ToolDefinition.SelectionTemplate and the SelectionPlaceholder marker that mcpgen puts into Query when the selection is templated.

Why

Static selection strings cannot express selection sets that depend on per-call argument values. For consumers like telemetry-api's signals query, where each {name, agg} pair needs to become its own sub-field name(agg: AGG), the current shortcut tool's selection timestamp returns empty buckets and forces models to stuff signal names into unrelated arguments like filter. Fixing this at the mcpgen layer means every consumer repo gets dynamic selections by annotation alone — no schema reshaping, no resolver rewrites, no breaking changes in consumer schemas.

Example

signals(tokenId: Int!, signalRequests: [SignalRequest!]!): [Bucket!]
  @mcpTool(
    name: "get_signals"
    description: "..."
    selection: "timestamp {{range .signalRequests}} {{.name}}(agg: {{.agg}}) {{end}}"
  )

A call with signalRequests: [{name: \"speed\", agg: AVG}, {name: \"rpm\", agg: MAX}] renders to:

query(...) { signals(...) { timestamp  speed(agg: AVG)  rpm(agg: MAX)  } }

Changes

• pkg/mcpserver/mcpserver.go: ToolDefinition.SelectionTemplate, exported SelectionPlaceholder, error propagation from registerShortcutTools through New.
• pkg/mcpserver/tools.go: parse each SelectionTemplate once at registration; render + splice per call; fail-fast on bad templates or missing placeholder.
• cmd/mcpgen/condensed.go: detect {{ in selection; emit SelectionPlaceholder in the generated Query; skip static validation for templated selections.
• cmd/mcpgen/generate.go: isTemplatedSelection helper; Go file template emits SelectionTemplate when non-empty.
• Tests:
  • cmd/mcpgen: TestTemplatedSelection, TestTemplatedSelectionGeneratedOutput + testdata/templated_selection.graphqls.
  • pkg/mcpserver: TestSelectionTemplateRendering, TestSelectionTemplateInvalidRejectedAtRegistration, TestSelectionTemplateMissingPlaceholderRejected.

Test plan

• make lint clean
• go test ./... (all existing + new tests pass)
• Bump server-garage in telemetry-api, convert the signals / signalsLatest @mcpTool annotations to templated selections, confirm the /mcp tools return correctly-shaped data end-to-end