fix(agents): slack context explicit-zero + gollem v0.27.0

[m-mizutani]

Follow-up to #226. Addresses review feedback on the typed-tool migration and picks up the tagged gollem release.

Changes

Fix: honor explicit before/after = 0 in the Slack context tool

agents/slack slack_get_context_messages regressed when the typed migration replaced the raw arg read with a > 0 guard: an explicit before: 0 (meaning "fetch nothing before, only after") was collapsed into the default of 10. A typed value field cannot distinguish an omitted argument from an explicit 0.

• Before / After are now *float64: nil (omitted) → default 10, a non-nil value (including 0) is honored as-is.
• The pointer is unwrapped to float64 for schema inference, so the wire type stays number — no schema change.
• Added TestInternalTool_GetContextMessages_ExplicitZeroSkipsSide covering the explicit-zero path.

(The > 0 defaulting for limit / count / page elsewhere is intentional — 0 / negative counts are meaningless there, so it is a robustness improvement, not a regression.)

Cleanup: bigquery config-tool constructor

newConfigGeneratorTools previously took a pre-built *configGeneratorTools and returned the same pointer ("a constructor taking its own output type"). It now takes the individual fields and constructs the value itself.

Deps: gollem → v0.27.0

Moves off the main pseudo-version pulled in by #226 to the tagged v0.27.0 release. The typed-tool / schema APIs are byte-for-byte identical to the pseudo-version, so this is a pin-to-release with no behavior change.

Test

• go vet ./..., go test ./... — all pass.
• golangci-lint run, gofmt -l — clean.

[gemini-code-assist]

Code Review

This pull request updates the gollem dependency to v0.27.0, refactors the Slack agent's context message tool to use pointer types for Before and After parameters to support explicit zero values, and adds a regression test. It also refactors the BigQuery helper's newConfigGeneratorTools constructor to accept individual arguments. The review feedback correctly points out that the tableDatasetID and tableTableID parameters are unused and suggests removing them to simplify the constructor.

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