fix: default PROMPT_EOL_MARK= in spawned shells

[ThomasK33]

What

Spawned shells now default PROMPT_EOL_MARK= (empty) in the session environment. This suppresses zsh's inverse-video % end-of-partial-line marker, which was leaking into snapshots, screenshots, and recordings.

Why

agent-tty injects a hidden completion-marker postamble after each run and strips that postamble's echo from the output stream. The strip leaves the rendered cursor mid-line, so zsh's unconditional end-of-line mark (PROMPT_EOL_MARK, default %B%S%#%s%b) surfaces as a stray standout % in captured artifacts. In a real terminal it's overwritten at column 0; in agent-tty's renderer it stays visible.

Before / after (echo hello snapshot):

❯ echo hello          ❯ echo hello
hello                  hello

❯ %          --->      ❯

❯                      ❯

How

A new pure, testable resolvePtyEnv(env, term, baseEnv) helper in src/pty/createPty.ts assembles the shell env with precedence (lowest→highest): inherited process env → PROMPT_EOL_MARK='' default → caller --env → TERM. The default is gated on 'PROMPT_EOL_MARK' in env (the user-explicit set), so a --env value always wins — even an explicit empty one — and is ordered after the inherited env so it also overrides an inherited value, keeping captures deterministic.

The marker is zsh-only and inert in other shells (bash, etc.), so it's safe to set unconditionally. The default is applied at PTY spawn time and is not written to the manifest, so inspect, list, and create --json env maps are unchanged.

Notes & caveats (documented in docs/USAGE.md / docs/TROUBLESHOOTING.md)

• Opt back in per session: agent-tty create --env PROMPT_EOL_MARK='%B%S%#%s%b' -- <shell> restores zsh's styled default. A lone '%' does not restore it (zsh treats it as a prompt escape that expands to nothing).
• A user ~/.zshrc that reassigns PROMPT_EOL_MARK runs after env import and wins — best-effort, documented.

Tests

• Unit (test/unit/pty/createPty.test.ts): default injected, caller value wins (incl. empty), overrides inherited, TERM forcing, undefined-drop.
• Integration (test/integration/lifecycle.test.ts): parent exports a sentinel PROMPT_EOL_MARK; asserts the spawned shell sees it empty — verified to fail without the fix and pass with it.
• Full suite green locally: 1196 unit, lifecycle 17/17, pty-basics + run 19/19; typecheck/lint/format clean.

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