fix(e2e): preserve Bub version when installing ACP

[Teingi]

Which issue or RFC does this PR close?

Closes #1822.

Rationale for this change

The harness installs Bub 0.4.2, but bub install bub-acp-server==0.0.2 initializes a plugin project with an unpinned Bub dependency and resolves it to 0.5.0. Its shell command path passes command, while ACP 0.0.2 expects cmd, breaking E2E acceptance before terminal execution. The failing master run reports this version drift in its ACP handshake and replay evidence.

What changes are included in this PR?

• Pass bub=={BUB_VERSION} together with the ACP plugin requirement so both PowerContext ON and OFF installations preserve the harness version.
• Document the version constraint across both installation steps and clarify that the local PowerContext plugin is installed when enabled.

Are there any user-facing changes?

E2E agent installation remains on the configured Bub version instead of silently upgrading it. There are no public API, persisted-format, or runtime dependency declaration changes.

How was this change tested?

• PATH="$PWD/.venv/bin:$PATH" TMPDIR=/data/codex-tmp uv run prek run -a — all hooks passed, including Ruff and the root type check.
• .venv/bin/ruff check e2e/bub and .venv/bin/ruff format --check e2e/bub — passed.
• .venv/bin/ty check --project e2e/bub --python e2e/bub/.venv --python-version 3.12 e2e/bub/src integrations/bub/src — passed.
• TMPDIR=/data/codex-tmp e2e/bub/.venv/bin/python -m pytest e2e/bub/tests — 101 passed.
• e2e/bub/.venv/bin/python -m powercontext_e2e --help — passed.
• Real isolated installs on Linux/Python 3.12.13 with uv 0.11.26: the original command upgrades Bub from 0.4.2 to 0.5.0; the fixed ON and OFF commands both retain Bub 0.4.2 and ACP 0.0.2. The ON environment also retains the local PowerContext plugin.
• A real ACP subprocess with PowerContext OFF reproduced the original argument validation/serialization failure. With the fix, a local JSON-RPC client successfully executed printf acp-pin-ok through the terminal and received end_turn.

The ACP subprocess probe ran outside the filesystem sandbox after both versions stalled inside it. This does not claim a complete Harbor, real-model, or SQLite/OceanBase acceptance rerun; those remain for CI.

AI usage statement

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