fix(shell): reject unsupported grep options

[aakarim]

Linear

Fixes OPE-32.

Amp work thread: https://ampcode.com/threads/T-01a0dc4c-c732-71c8-a179-937af4910847

Diagnosis

CmdGrep manually parsed short-option groups but had no default case, so unsupported short flags were silently discarded. Arguments beginning with -- bypassed option parsing and became the search pattern. As a result, repeatable -e shifted subsequent patterns into file-path position and produced misleading missing-file errors.

Fix

• Reject unsupported short and long options with exit code 2 and an error naming the option.
• Direct callers using repeatable -e to the supported -E 'a|b' form.
• Add regression coverage for standalone/grouped short flags, long flags, and repeatable -e.
• Align interactive help and the command reference with the options actually supported.

Verification

• go test ./pkg/shell/cmds -run '^TestGrep'
• go test ./...
• go vet ./...
• git diff --check

All checks pass in the repository's Nix development shell.

[linear-code]

OPE-32

[Copilot]

Copilot review overview

🟡 Changes recommended

Align assets/lore/commands.md with the updated grep options.

Review effort: Lite
Findings: 1

Open (1)

• Embedded command reference remains stale after grep flag changes · New

What changed in this PR

This PR makes grep reject unsupported options with exit code 2 and updates related guidance.

Changes:

• Adds validation for unsupported short and long options.
• Adds regression tests for invalid options and repeatable -e.
• Updates interactive help and command documentation.

File	Summary
pkg/​shell/​cmds/​help.go	Updates interactive grep help.
pkg/​shell/​cmds/​grep.go	Validates grep options.
pkg/​shell/​cmds/​grep_test.go	Adds regression coverage.
docs/​commands.md	Updates the command reference; the embedded reference still needs alignment.

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