Epic: retire the deepseek-tui-era identifiers — tiered migration (internals now, env aliases deprecated to 0.10, storage keys via explicit migration)

[Hmbown]

Why tiered

AGENTS.md: compatibility identifiers and storage keys change only through an
explicit migration. Some DEEPSEEK_* identifiers are the provider's name and
stay (DEEPSEEK_API_KEY as provider credential — ANTHROPIC_API_KEY-shaped).
The rest split by blast radius.

Tier 0 — keep (provider identity, not era)

• DEEPSEEK_API_KEY where it means the DeepSeek provider credential
  (crates/tui/src/dsh_credentials.rs:11,144; provider auth in setup).

Tier 1 — internal renames, land in 0.9.9 (zero user impact)

• handle_deepseek_turn → run_turn (core/engine/turn_loop.rs:512; call
  sites engine.rs:4222 + 9 in engine/tests.rs; stale doc refs in
  crates/core/src/engine/thread/{mod,executor}.rs, dispatch.rs)
• pi_delegate/pi_timeout (tools/shell.rs:3792-3799) + pi_* edit helpers
  (tools/file.rs:1578-1775) — v0.9.6 vocabulary; coordinate with the
  one-bash consolidation design
• SubAgentRuntime::step_api_timeout doc ("Per-step DeepSeek API timeout")

Tier 2 — env aliases: CODEWHALE_ primary + deprecation warning in 0.9.9, delete at 0.10.0

Template: CODEWHALE_STREAM_IDLE_TIMEOUT_SECS (subagent_limits.rs:56-58) already
carries DEEPSEEK_STREAM_IDLE_TIMEOUT_SECS as a legacy alias.

• DEEPSEEK_MODEL / DEEPSEEK_DEFAULT_TEXT_MODEL (config/src/lib.rs:6737 chain)
• DEEPSEEK_PREFER_BWRAP — and flip config.example.toml:378 to document
  CODEWHALE_PREFER_BWRAP as the override
• DEEPSEEK_CONFIG_PATH (ui.rs:3179-3205, startup_defaults.rs:872,919, views/mod.rs:5166)
• DEEPSEEK_SESSION_ID (ui/provider_routes.rs:17)
• DEEPSEEK_TOOL_CALL_ID (tui/tool_routing.rs:1822)
• DEEPSEEK_BUILD_VERSION (tui/underwater.rs:323)
• DEEPSEEK_RUNTIME_HOST / DEEPSEEK_RUNTIME_TOKEN (tui/setup/remote.rs:30,35)
• DEEPSEEK_SANDBOX child-env marker (sandbox/*) — observable by user
  scripts; keep as alias longest
• Mechanism: each alias read logs one deprecation line naming the new
  spelling; CHANGELOG "Deprecations" section each release until 0.10.0 removes.

Tier 3 — storage keys: read-migration + one-time notice in 0.9.9, delete at 0.10.0

• ~/.deepseek config-home fallback (config/src/lib.rs:5828,5873)
• ~/.deepseek/sessions legacy migration (session_manager.rs:2895+ — exists)
• .deepseek/instructions.md (core/src/fragments.rs:259)
• .deepseek/handoff.md (tui/src/prompts.rs:83)
• Each: keep read-path, add a one-time "found legacy ~/.deepseek — migrated"
  notice where missing, remove in 0.10.0. bwrap protected-descendant lists
  keep guarding .deepseek paths while any migration reads them.

[Hmbown]

Rolled to v0.9.10: the v0.9.9 release branch merged to main tonight (#5476) without this landing; nothing here is closed or dropped.