fix(ca): fall back to the default project when a directory link is stale

[codyde]

Problem

railway code --railway (and every other railway code / railway ca launch) failed with

Project not found. Run railway link to connect to a project.

when run in a directory whose link points at a project that has since been deleted. Two things made this baffling in practice:

• The link lookup walks ancestor directories, so the stale link is often in a parent the user doesn't remember linking (e.g. ~/projects linked to a long-deleted sandbox project poisons every launch under it).
• The user's expectation — "no usable link → use my default cloud agents project" — is exactly what the precedence in choose_target is meant to deliver, but the link was trusted without validation. resolve_target returned the dead IDs, the pane launch re-entered the pipeline via LaunchArgs::for_target (flags), and get_project finally hit the API and died with the generic error above.

Fix

Probe the link in resolve_target before it outranks the configured default. A definite server-side "gone" demotes it:

• project not found
• project readable but deletedAt set
• linked environment missing or deleted

with a warning naming what went stale (This directory is linked to a project that no longer exists — ignoring the link (railway link to fix it).) plus which default the launch is using instead. The launch then lands where a linkless run would have: the configured default, or railway ca setup when there is none.

Anything short of definite (auth/network errors) keeps the link and lets the launch's own calls decide — a transient failure must not be able to reroute a launch to a different project.

Flags (-p/-e) are unaffected: an explicitly passed dead ID still errors, as it should. The probe only runs when a link would actually be consulted (no flags), so flag-driven and TUI launches pay nothing.

Notes

• The staleness verdict lives in a pure stale_link_reason helper with unit tests covering all verdicts (not found / deleted project / missing env / deleted env / healthy / inconclusive).
• Known follow-up, deliberately out of scope: railway ca ssh <name>'s first-agent auto-create consults the link directly (lifecycle.rs) and would still target a stale environment; and a configured default that is itself dead still fails with the generic error (setup re-validates it).

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