exec: forward pty resize end-to-end (runtime + CLI)

[bilby91]

Fixes terminal-resize misbehavior in `devcontainer exec`: interactive programs render at 80x24 until first redraw and ignore subsequent window resizes.

Two commits:

1. `runtime: forward pty resize through ExecContainer`

Adds `InitialTtySize` and `ResizeCh` to `runtime.ExecOptions` (mirrored on `devcontainer.ExecOptions`).

Docker backend:

• `ConsoleSize` passed to `ExecCreate` so the pty starts at the right geometry
• Small goroutine drains the resize channel into `ExecResize` daemon calls, coalescing bursts (keeps only the latest size when several updates arrive between API round-trips) so a fast drag doesn't back up

Apple-container backend: out of scope here; falls back to the no-resize behavior it already exhibits.

2. `cli: forward SIGWINCH into exec resize channel`

`setupTty` becomes a `ttyState` struct carrying:

• `initial` — `term.GetSize` at exec start, passed as `InitialTtySize`
• `resize` — channel fed by a `SIGWINCH` listener goroutine, passed as `ResizeCh`
• `restore` — cancels the listener + restores cooked-mode stdin

The listener goroutine binds its lifetime to a context cancelled by `restore`, so it exits cleanly when `Exec` returns or the user `Ctrl-C`s.

Test plan

• `go build ./...`, `golangci-lint run`, linux cross-build all clean
• Rebuilt local binary, ran `devcontainer exec -- htop` (or any curses app), confirmed it renders at the host terminal size and reflows when the window is resized
• Apple-container resize plumbing — follow-up (would mirror the docker goroutine through the Swift bridge)

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Enhanced terminal support for command execution with automatic detection of terminal dimensions and dynamic resizing capabilities during execution.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 5e74604d-7436-4f1e-b744-656cc4529f93

📥 Commits

Reviewing files that changed from the base of the PR and between 3b9ff57 and 29c6b21.

📒 Files selected for processing (4)

• cmd/devcontainer/exec.go
• exec.go
• runtime/docker/exec.go
• runtime/runtime.go

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📝 Walkthrough

Walkthrough

This PR adds end-to-end support for terminal size initialization and dynamic resizing in exec operations. The runtime API gains a TtySize type and ExecOptions fields for initial size and a resize channel. The CLI command captures terminal dimensions and SIGWINCH signals via a new setupTty helper, and the Docker runtime integrates those dimensions into exec creation and handles ongoing resize events forwarded through a channel.

Changes

TTY Size and Resize Support

Layer / File(s)	Summary
Runtime TTY size types and options runtime/runtime.go	Adds TtySize struct with Width and Height fields; extends ExecOptions with InitialTtySize and ResizeCh fields, both documented to apply only when Tty is true.
Engine exec option forwarding exec.go	Extends ExecOptions with TTY size fields and forwards them into the runtime ExecOptions passed to ExecContainer.
CLI terminal resize handling cmd/devcontainer/exec.go	Updates imports for signal and syscall support; initializes setupTty in the exec command's RunE for error handling and deferred cleanup; passes InitialTtySize and ResizeCh to the engine; implements setupTty to enter raw mode, capture initial terminal size, listen for SIGWINCH events, and forward size updates through a channel until context cancellation.
Docker runtime console size and resize runtime/docker/exec.go	Computes a Docker ConsoleSize from InitialTtySize when TTY is enabled and dimensions are non-zero, wires it into ExecCreateOptions, and starts a resize-forwarding goroutine that coalesces updates from the resize channel and applies them via ExecResize for the lifetime of the exec. Includes forwardExecResize helper to drain and apply the latest resize event while respecting context cancellation.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

A rabbit hops through signals bright,
SIGWINCH whispers in the night—
Terminal sizes flow like streams,
Resize channels realize dreams,
TTY tamed, the shell feels right! 🐰✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: implementing end-to-end PTY resize forwarding across the runtime and CLI components, which aligns directly with the PR's core objective of fixing terminal-resize behavior.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cli/tty-resize

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