fix: add --session-id to codex, remove --discover latest from all import commands

[christso]

Summary

• Adds --session-id <uuid> to agentv import codex for parity with claude and copilot
• Removes --discover latest from all three import commands — redundant since --list shows sessions in descending order and --session-id imports a specific one
• Updates all docs (import.mdx, running-evals.mdx, guides, SKILL.md, example README)
• Adds codex and copilot to the supported providers table in import docs

Closes #952

Before

agentv import claude --discover latest        # auto-import most recent
agentv import codex --discover latest         # only option (no --session-id)

After

agentv import claude --list                   # see sessions
agentv import claude --session-id <uuid>      # import one

agentv import codex --list
agentv import codex --session-id <uuid>       # new

agentv import copilot --list
agentv import copilot --session-id <uuid>

Test plan

• Build passes
• Typecheck passes
• Lint passes
• All tests pass
• agentv import codex --list shows sessions
• agentv import codex --session-id 019d5cff-9f02-7bc3-8f98-2071ba17ef0e imports correctly (55 messages, 26 tool calls)
• agentv import codex (no args) shows helpful error: "specify --session-id ... Use --list to see available sessions"

🤖 Generated with Claude Code

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