internal/context: delete the deprecated alias layer; update all calle…

[wolo-lab]

…rs to import agent directly

Sweeps the 28 importers of internal/context (alias 'icontext' or 'contextinternal') to use agent.NewX directly. The alias layer introduced in PR #1 was a transitional stage; with PR #2 (unified agent.Context interface) landed, no consumer needs internal/context any more.

Mechanical changes:

• icontext.NewInvocationContext → agent.NewInvocationContext (most uses)
• icontext.InvocationContextParams → agent.InvocationContextParams
• icontext.NewCallbackContext → agent.NewCallbackContext (2 uses)
• icontext.NewCallbackContextWithDelta → agent.NewCallbackContextWithDelta (8 uses)
• icontext.NewReadonlyContext → agent.NewReadonlyContext (3 uses)

Per-file: 28 importer files lose their icontext "…/internal/context" import line. internal/toolinternal/context_test.go had a different alias name (contextinternal); also rewritten.

One ad-hoc fix: agent/remoteagent/v2/utils_test.go had a local variable named agent shadowing the package name; renamed to a (unrelated to the sweep but exposed by it).

Then deletes internal/context/ entirely. The package no longer exists; agent is the single home for canonical context constructors.

Verified: go build ./...; go vet ./...; go test ./...; all pass.

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