feat: shell passthrough context injection and real-time streaming

[emal-avala]

Summary

Upgrades the ! prefix shell command (repl.rs:603) with two missing capabilities:

• Real-time streaming: Switches from .output() (waits for completion) to Stdio::piped() with line-by-line BufReader output. Long-running commands now stream to the terminal as they execute.
• Context injection: Captures stdout+stderr into a buffer and injects it into conversation history as a UserMessage with is_meta: true. The agent can reference shell output in subsequent turns without the user copy-pasting or using the Bash tool.
• 50KB truncation: Prevents context bloat from large outputs (verbose test suites, build logs). Appends [output truncated at 50KB] when the limit is hit.

Before

> ! cargo test
(waits... waits... then dumps all output at once)
(agent has no idea what the output was)

After

> ! cargo test
running 45 tests              ← streams in real-time
test auth::test_login ... ok
test billing::test_charge ... FAILED

> The billing test is failing — can you see why?
(agent has the test output in context, can analyze it immediately)

What doesn't change

• No LLM tokens consumed (not a turn)
• No permission check needed (user explicitly invoked the command)
• The ! prefix syntax is unchanged

Implements roadmap item 7.18.

Test plan

• cargo check — clean compile
• cargo clippy -- -D warnings — zero warnings
• cargo test — all 232+ tests pass
• Manual: run ! echo hello and verify output appears in real-time
• Manual: run ! ls then ask agent "what files did you see?" — verify it references the output
• Manual: run a command producing >50KB output, verify truncation message appears

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