v0.8.56: Fix rebrand update path — `deepseek update` and npm updates fail to reach codewhale

[Hmbown]

Goal

Make the upgrade path from the pre-rebrand install (deepseek-tui / old npm package) to current codewhale releases work, or fail with clear, copy-pasteable migration instructions. Users on old installs currently hit hard errors with no guidance.

Current evidence

• Cargo distribution: error: failed to spawn `codewhale' after changed from deepseek-tui #2917: a Cargo-installed deepseek-tui user ran deepseek update and got error: failed to spawn \codewhale`: codewhale not found on PATH or in sibling directory` — the old binary's self-update now assumes the new binary name exists.
• I can't update to the new version using npm. #2924: an npm user reports being unable to update to the new version at all (details sparse, but consistent with the package rename).
• release: harden Cargo and npm publish recovery checklist #2643 covers release-side publish recovery, but nothing tracks the user-side update regression from the rename.

Scope

• Audit the self-update code path for what happens when the running binary is the legacy name and codewhale is not installed: either perform the migration (install codewhale-cli / new npm package, then hand off) or print exact migration commands per install method (cargo, npm, Homebrew, release binary).
• Verify the npm rename path: deprecation notice on the old package pointing at the new one, and confirm npm update/npm install -g of the old name surfaces the migration message.
• Add a short migration section to the README/docs covering cargo and npm upgrades from pre-rebrand versions.
• Test: simulate the legacy-binary update flow and assert the user gets either a successful migration or actionable instructions, never a raw spawn error.

Non-goals

• Republishing or maintaining the legacy package long-term.
• Broader release-process hardening (tracked in release: harden Cargo and npm publish recovery checklist #2643).
• State/config migration from ~/.deepseek (already handled in P0 v0.8.44: migrate app state to ~/.codewhale with ~/.deepseek compatibility bridge #2011 and 问题：程序更名成code whale之后，原先的会话、技能都还在吗 #1969/TUI: fix large-paste .deepseek writes and long status readability #2914 follow-ups).

Acceptance criteria

• deepseek update on a machine without codewhale no longer dies with a spawn error; it migrates or prints exact per-install-method instructions.
• The npm upgrade path from the old package name to the new one is verified and documented.
• Docs include a 'Upgrading from deepseek-tui' section.
• Cargo distribution: error: failed to spawn `codewhale' after changed from deepseek-tui #2917 and I can't update to the new version using npm. #2924 can be closed against this work.

Related: #2917, #2924, #2643, #2011, #1969