[keenser]

Description

Adds provider-less terminal connections to the extension system. A connection contribution can now declare a terminal block (command, args, env, title) instead of a provider runtime. When the user opens such a connection, the host starts a regular local terminal tab from the declared template, using the non-secret values saved in the connection form.

Problem: an extension whose only job is "run a local command with a few saved parameters" (e.g. tsh ssh <target> for Teleport) still had to ship and spawn a native provider process plus a resource workbench. Sessions are tied to a live provider client, so this could not be simplified without a large rework. With this change, such an extension is a single extension.json with no runtime, no bin/, and no Rust.

What changed:

• Manifest / registration (extension-runtime): runtimeId and resourceType become optional. terminal is mutually exclusive with runtimeId, resourceType, shellViewId, and workbench bindings. command is fixed (no placeholders). Placeholders ({{fieldId}})
  in args, env values and title may only reference existing non-secret form fields. Secret fields are rejected for terminal connections.
• Launch resolution (terminal_launch.rs): substitutes placeholders from the saved connection values. Optional args/env entries that resolve to an empty string are dropped. Control characters in values are rejected.
• Option-injection guard: without a provider there is no per-extension input validation, and connection data may come from cloud sync / team sharing. If an argument template does not start with - but the substituted value does, the open is rejected (e.g. target = "-oProxyCommand=..." cannot become an ssh option).
• Terminal tab (terminal, terminal_view): launched sessions get a fixed tab title, can be duplicated, and restart on Enter after the child process exits instead of leaving a dead tab.
• Host open path (home_strategy.rs): new branch that opens a terminal tab for contributions with terminal, independent of the shell-plugins feature.
• Connection form: the "Test" button is hidden for terminal connections (there is no provider to probe).
• Docs: new section in the extension development guide.

The companion extension (tsh-ssh, static composite, engines.onetcli >= 0.20.3) lives in navop-extensions and is submitted separately. The submodule pointer is intentionally not part of this PR.

Break Changes

No behavior change for existing extensions. Two API-level notes:

• ResourceConnectionContrib.runtime_id / resource_type now default to empty on deserialization. Registration still rejects a non-terminal connection that leaves them empty, with the same error as before.
• RegisteredResourceConnectionContribution gains a public terminal: Option<ResourceConnectionTerminal> field, so Rust code constructing it by struct literal must add terminal: None.

 RegisteredResourceConnectionContribution {
     ...
     shell_view_id: None,
+    terminal: None,
     form: Default::default(),
 }

How to Test

Automated:

cargo test -p extension-runtime --lib -- terminal # template resolution, option-injection guard, registration rules
cargo test -p terminal_view --lib duplicate # duplicate source for launched terminals
cargo test -p terminal --lib restart # restart-after-exit for local sessions
cargo test -p main home
cargo check --workspace --tests

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)