…R #1)

Same goal as wolo/context_unif_v2_pr1_move (eliminate the 3 pairs of duplicate context impls + the duplicate internalArtifacts helper), opposite execution: keeps existing implementations in agent/agent.go instead of moving them to new files.

What's added to agent/agent.go (~146 lines):

• internalArtifacts wrapper (delta-tracking for Save).
• NewCallbackContext / NewCallbackContextWithDelta constructors.
• artifacts field on callbackContext + updated Artifacts() to return the wrapper (bug fix: previously returned unwrapped Artifacts, so Save() in callbacks didn't record into EventActions.ArtifactDelta).
• NewInvocationContext + InvocationContextParams.
• readonlyContext struct + 8 methods + NewReadonlyContext.
• InvocationOf helper (returns underlying InvocationContext from a NewReadonlyContext-produced ReadonlyContext).
• runBeforeAgentCallbacks / runAfterAgentCallbacks switched from raw struct literal to newCallbackContextImpl (no behavioural change — the new bug fix takes effect either way).

What's removed:

• internal/context/{invocation,callback,readonly}_context.go shrink to thin alias layers (var NewX = agent.NewX).
• internal/context/context_test.go deleted (its TestWithContext sibling already exists in agent/agent_test.go; the other two tests can be re-added in agent/ if desired).
• internal/toolinternal.internalArtifacts dropped (toolContext now relies on the embedded CallbackContext for Artifacts wrapping — eliminates double-wrapping bug).
• util/instructionutil drops the icontext import; uses agent.InvocationOf instead of *icontext.ReadonlyContext type assertion.

Trade-off vs the canonical PR #1:

• agent/agent.go grows from 521 to ~660 lines (mixing Agent + Run
  • Context impls + helpers in one file).
• 7 files touched instead of 13; +168/-352 = -184 lines net, versus +489/-537 = -48 net for PR Bump the go_modules group across 1 directory with 2 updates #1. ~136 lines smaller diff.
• No new files in agent/ (3 fewer in git status).

This branch exists for comparison; whichever direction we pick gets PRs #2 and #3 stacked on top.

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

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