Agent mode not detected under Claude Code; explicit --agent-mode dropped

## Bug

Two related agent-mode defects (both needed for agent mode to work):

1. **Auto-detection never fires under Claude Code.** The agent-env list has `CLAUDE_CODE`, but Claude Code actually sets `CLAUDECODE` (no underscore), so agent mode — and its TOON output default — never activates.
2. **An explicit `--agent-mode` is dropped.** `Execute()` calls `InitAgentMode` once *before* flag parsing (to gate the explorer TUI). Its `CompareAndSwap` latch makes the later post-parse call in `PersistentPreRunE` a no-op, so `--agent-mode` / `--agent-mode=false` on the command line is ignored.

## Root cause (Speakeasy-generated code)

1. `internal/output/agentmode.go` — `agentEnvVars` is missing `CLAUDECODE`.
2. `internal/cli/root.go` — `Execute()` runs early pre-parse detection to gate the TUI; the latch in `InitAgentMode` then blocks re-evaluation after flags are parsed.

## Stopgap fix (retire when upstream is fixed)

- `.speakeasy/patches/internal/output/agentmode.go.patch` — adds `CLAUDECODE` to `agentEnvVars`.
- Hand-written `internal/cli/custom_startup.go` — the chained post-flag-parse `PersistentPreRunE` calls `output.ResetAgentMode(); output.InitAgentMode(cmd)` when `--agent-mode` was explicitly changed, so the flag wins over the early latched detection.

**To retire:** once Speakeasy (1) includes `CLAUDECODE` in its agent-env list — remove the `agentmode.go.patch` hunk; and (2) re-evaluates agent detection after flag parsing so an explicit flag wins — remove the re-eval block from `custom_startup.go`.

## Refs

- Introduced by #14
- Upstream generator bug — should also be filed to speakeasy-api/speakeasy. (The `CLAUDECODE` addition is trivially upstreamable.)

Start with internal/output/agentmode.go and internal/cli/root.go, then trace Execute(), InitAgentMode, and PersistentPreRunE to understand the pre-parse and post-parse sequence. Verify that Claude Code detection works and that explicit --agent-mode and --agent-mode=false take precedence; done means the stopgap patches can be retired without either behavior regressing.

Written by the indexing model from the issue text.