…#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 #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.